New & Noteworthy

Exploding Yeast to Protect the Wild

January 22, 2018


In the first Kingsman movie, Samuel L. Jackson plants little bombs into the necks of people who will follow him to his “ark”. If they disobey him, Jackson can simply blow up their heads.

In an amazing scene from the movie, our heroes get a hold of the “detonator” and blow up the heads of every one of Jackson’s followers. As you can see below, the director uses cool colors to make it more fabulous than gruesome:

In a new study out in Nature Communications, Maselko and coworkers explode misbehaving yeast instead of people’s heads. (Click here for an incredibly cool video of it happening…so cool!)

These researchers have created a strain of yeast that can only form diploids within the same strain. If instead it mates and forms a diploid with a yeast outside of its strain, the resulting diploids explode.

These authors aren’t being strainist—they are creating a proof of principle organism that could potentially solve the problem of gene transfer from genetically modified organisms (GMOs) to wild organisms.

Assuming a similar idea works in plants, then the idea is that a GMO like an herbicide-resistant GM plant could not pass its herbicide-resistance to its wild cousin. The resulting plants would explode (or perhaps die in a bit less spectacular fashion). The herbicide-resistant gene would not escape into the wild population because none of the resulting plants could survive.

And that isn’t this approach’s only possible uses. It might be used to kill off disease carrying mosquitoes or controlling invasive species. Watch out sea lamprey!

The basic idea is that the engineered species makes a “poison” that it is immune to. If it mates with a wild species, it passes that poison on to the resulting “children.” These progeny are not immune and so die.

The poison in this case is a CRISPR/Cas9-inspired transcription activator. This activator turns a gene up so much that it kills the yeast. But the catch is that the activator’s binding site is only found in the wild type genome and not the engineered genome.

So the engineered strain carries around this deadly poison that does not affect it. But when this yeast mates with a strain that has the binding site, the resulting diploid yeast has the activator binding site, meaning that the silent killer can now bind and kill the diploid.

For their proof of principle experiments, Maselko and coworkers used a well-tested and available CRISPR/Cas9 transcriptional activator that involves dCas9-VP64 and a single guide RNA aptamer binding MS2-VP64. Previous work has shown that this is a very powerful activator.

Next they turned to the Saccharomyces Genome Database (SGD) to find genes that have been shown to be lethal when overexpressed. They created at least one guide RNA for each of the 8 genes and found that one of the guide RNAs that targeted the ACT1 promoter was lethal. The yeast made actin until it exploded (think the man in Monty Python’s Meaning of Life who eats so much he explodes).

Now they were ready to create their engineered strain. They used CRISPR/Cas9 to engineer a single nucleotide change such that their activator could no longer bind near the ACT1 gene. This strain survived when the activator was introduced.

But when they mated this strain with a wild type strain, they got almost no survivors. The resulting diploids grew fat on overproduced actin and exploded!

Note that there were a few survivors…as Jeff Goldblum famously said in Jurassic Park, “Life will find a way.” The survivors appeared at a frequency of 4.83 × 10−3 with the most common reason found being a mutation in the activator binding site.

This survival rate is one area of obvious concern. Given the genetic variability of wild populations, there may be a few individuals that happen to have a mutation that is resistant to the activator. These immune individuals would then go on to found a new population.

Hawat

Like the engineered yeast in this strain, Hawat can survive only because he has both the poison and the antidote. (Dunepedia)

The authors suggest that targeting a species that has gone through a recent bottleneck to decrease genetic variability might help. Another possibility might be to target multiple genes making the likelihood of an individual having and/or developing multiple mutations exceedingly unlikely. (This is the strategy used to target HIV, a virus with a high mutation rate.)

These authors have created an elegant system reminiscent of the one created by Baron Harkonnen in the novel Dune. In the book, a character named Thufir Hawat is given a slow acting poison by the Baron. Hawat can be controlled because he needs the antidote each day to survive. Once the antidote is removed, he dies.

Here the researchers have given the poison and engineered the antidote into their new strain. When this strain mates with a strain lacking the antidote, like a wild strain, most of the progeny die. Like Hawat, the yeast die for the good of mankind. And hopefully those pesky weeds can be prevented from picking up transgenes too.

by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: synthetic biology, horizontal gene transfer, CRISPR

Many Modules Make Light(er) Work

August 26, 2015


If you’ve ever put together something from IKEA you know it can be a bear. So many parts need to connect up together perfectly to build that new bookcase—if you tried to do it all at once you’d go crazy.

Complicated tasks are way easier to do if they are broken up into smaller chunks. This is true whether you are building a bookcase or a biochemical pathway. Image via Wikimedia Commons

Luckily the good folks at IKEA try to make it a bit easier (and more tolerable) by splitting the task into smaller, more manageable pieces. You can concentrate on one part without having to worry about the rest. Once that part is done you can work on the next part and so on. In the end you assemble all the pieces together into your new bookcase.

This is the approach Galanie and coworkers took in their recent Science paper where they engineered our favorite yeast S. cerevisiae to make a couple of different opioids. And it is a good thing they broke this problem up, because it was a way bigger undertaking than anything IKEA might have thrown at them. Engineering this yeast strain was a genetic tour de force.

The authors coordinated 21 different genes from mammals, plants, bacteria and yeast to get the opiate precursor thebaine made. And the semisynthetic opiate hydrocodone took an extra two genes for a grand total of 23! Trying to do all of these at once might have been very frustrating. Thank goodness they split this Herculean task into six (or seven for hydrocodone) smaller modules.

The first step was to get yeast to make (S)-reticuline, a key intermediate on the way to useful opiates. This took 4 modules made up of 17 different genes: six from rat, six from plants, four from yeast and one from bacteria.

And of course just putting these into yeast all at once would almost certainly have made a whole lot of nothing. Each gene needed to be selected from the right beast and then optimized to work in the yeast strain. Sometimes this meant picking the right variant from the right plant, and sometimes it meant mutating a gene to make it behave better. This all would have been overwhelming if the task weren’t split into four easier sections.

Even with all of their optimization, this iteration only made about 20 μg/liter of (S)-reticuline. They needed yeast to crank out more of this intermediate, so they designed a fifth module.

As its name implies, this “bottleneck” module was designed to overcome bottlenecks in the first four modules. After it was added to the strain, the yeast managed to make 82 μg/liter. This was something they could work with!

Except now they were stuck. They needed (R)-reticuline instead of the S form, but no one knew how poppies managed this feat. The gene that did this job hadn’t yet been discovered.

So Galanie and coworkers rolled up their sleeves and dug through plant transcriptome databases to find the gene they were looking for. They found a likely candidate, synthesized the gene in order to produce the enzyme, tested whether it could transform the S form of reticuline into the R form in vitro, and found that it could.

They could now make the right intermediate, which meant they could make their final module. As its name implies, this “thebaine” module would finally allow them to make the opiate precursor thebaine in yeast. This module consisted of their recently discovered gene and three other plant genes.

They had finally made thebaine from simple sugars in yeast! Except it didn’t work very well at all. There seemed to be a bottleneck right after the (R)-reticuline stage. Back to the drawing board!

Given where the bottleneck was, the researchers guessed correctly that the culprit was the SalSyn enzyme which converted (R)-reticuline to salutaradine. A Western blot showed three distinct forms of this enzyme in yeast and only one form, the lowest molecular weight one, when it was expressed in tobacco. Clearly something was happening to inactivate this protein in yeast.

A close look at the protein suggested yeast was glycosylating positions that it shouldn’t, and site directed mutagenesis of these sites confirmed this. The glycosylation was causing the protein to be sorted incorrectly so that it couldn’t do its job.

Unfortunately just mutagenizing away the glycosylation sites wasn’t good enough, because this severely affected the enzyme’s ability to do its job. So the researchers created a chimeric protein with parts of another P450 enzyme they knew did great work in yeast. After optimizing its codons for yeast, this chimera performed beautifully.

Now, finally, they had a yeast strain that could make thebaine. Not a lot of it, only around 6.4 μg/liter—but amazing nonetheless.

A yeast strain would have to be millions of times better at making opioids before a Walter White character could turn it into a profitable criminal activity. But the authors advocate for starting an open dialog on synthetic biology issues now, while there’s still time to deliberate. Image by Hecziaa via deviantart.com

A final module was added that consisted of two plant enzymes that converted thebaine to the drug hydrocodone. This monster strain could crank out around 0.3 μg/liter of hydrocodone. Yes, that is as puny as it sounds; one dose of painkiller for an adult would contain 5 mg of hydrocodone.

To be competitive with poppies, they need a 100,000-fold improvement to around 5 mg/liter. In talking with Dr. Smolke, it sounds like this could happen within a couple of years. After scaling up for production, voila! An entirely new source of opiates for pain relief.

Of course the elephant in the room is a Breaking Bad-esque scene where a yeast biologist grabs ahold of an opiate-producing strain and supplies various cartels with illegal drugs. Our Walter White wannabe wouldn’t be able to use the current strain, as he would need thousands of liters of yeast to produce a single dose of Vicodin.

But this scenario will be a real concern in the next few years. Which is why the Smolke lab has crossed every t and dotted every i in setting up and creating this strain. They have made it as difficult as possible for the wrong people to get their hands on it.  

This strain represents a stunning achievement in synthetic biology. Move over poppies, there’s a new opiate producer in town.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: pathway engineering, opiate biosynthesis, Saccharomyces cerevisiae, synthetic biology

Redesigning Life, Ethically

August 13, 2015


Yeast is an essential ingredient in the recipes for our most delicious food and drink. But now, researchers are working on a recipe for yeast itself! Image by Maria Costanzo

For thousands of years, humans have used yeast as an essential part of recipes for bread, wine, and beer. But now we’re turning the tables on yeast. Instead of creating recipes with yeast, researchers are creating recipes for yeast.

Now of course we have been making minor tweaks here and there for years. But what we’re talking about now is changing out the whole recipe book: creating a whole new genome for S. cerevisiae.

The Synthetic Yeast 2.0 Project (Sc2.0) has the ambitious goal of re-designing and synthesizing the entire yeast genome, some 12 million base pairs. Along with the scientific challenges, the researchers face some tricky ethical issues as well. After all, they’re creating the blueprint for an entire living eukaryotic cell!

Fortunately, the Sc2.0 researchers have thought long and hard about these issues. They’ve issued a statement of ethics and governance in a new article in GENETICS that also reviews the current regulatory and ethical landscape for synthetic biology. The statement by Sliva and colleagues sets the course for Sc2.0 and serves as a model for oversight of other synthetic biology projects.

We wrote about the science in this space before, when the project published its first major milestone, the synthesis of chromosome III. It’s fascinating stuff: the scientists are not only re-synthesizing the genome, but are re-designing it to be leaner and more useful in the lab. They’re adding features like loxP sites that can be used to alter the structure of the genome for evolution experiments, and engineering the tRNA genes so that one codon can be repurposed to code for a novel amino acid.

But even though these are seemingly benign changes to a relatively harmless beast, there are ethical issues inherent in modifying a living organism in such a big way. While the authors focus on Sc2.0, the issues they discuss are relevant to other synthetic biology projects that combine genes from several organisms in novel pathways, such as the efforts to create an opiate biosynthetic pathway in yeast

While we can only touch on the highlights of their statement here, one of the principles most strongly emphasized by Sliva and colleagues is that all Sc2.0 work will be done for peaceful purposes that benefit society. To promote transparency, they are making outreach a priority, engaging with and educating the public about the project. Sc2.0 will be a public resource, with no intellectual property rights or restrictions on data or materials.

The researchers are also committed to safety. They have engineered multiple auxotrophies into all working strains so that they need special media to survive, even though it seems unlikely that an Sc2.0 strain on the loose would be harmful or would have a competitive advantage over wild strains. And although it’s not required for working with organisms like yeast that are classified “Generally Regarded as Safe” by the FDA, all participants in the project receive biosafety training.

Currently, there is relatively little official policy in place for the field of synthetic biology. Two safety measures currently recommended for DNA synthesis companies by the U.S. Department of Health and Human Services are that the companies check that the sequences they synthesize don’t correspond to toxins or harmful organisms, and that they also verify the identities and institutions of their customers. While compliance with these guidelines is voluntary, the Sc2.0 project has decided to support only companies that follow these safety measures.

To ensure that the policies outlined in their Statement of Ethics and Governance are followed, the Sc2.0 project will maintain an Executive Committee comprised of people both internal and external to the project who have broad expertise in policy, ethics, and science. All of the participants in the project are accountable to this committee, which will actively monitor the work to ensure that the guidelines are followed.

It’s obvious that this is no half-baked scheme, but rather an impressively well-planned recipe for cooking up a yeast cell from scratch. But, we expect nothing less from our friend S. cerevisiae and the talented researchers in the yeast community than to be at the forefront of modern science! 

by Maria Costanzo, Ph.D., Senior Biocuration Scientist, SGD

Categories: Research Spotlight

Tags: synthetic biology, Saccharomyces cerevisiae

Red Ink for the S. cerevisiae Genome

April 02, 2015


Text editing has come a long way since the fountain pen, and genome editing is now almost as easy, thanks to the CRISPR/Cas system. Image by Nic McPhee via Flickr.com

Editing is an essential part of producing good writing. You might cringe when your masterpiece comes back covered in red ink, but in the end your paper is better for it.

These days, computers have made editing much, much faster and easier than using ink on paper. Some of us remember when cutting and pasting literally mean cutting the piece of paper on which your manuscript was typed and pasting the sections in a different order!

The same kind of revolution has happened when it comes to editing genomes. A seemingly obscure system used by bacteria to defend against invading phages, discovered in the 1950s, led to the development of restriction enzymes as the reagents that have enabled virtually all modern molecular biology. And now, an equally “obscure” system of bacterial immunity has opened the door to genome editing that is as precise and nearly as fast as your thesis advisor using Microsoft Word to edit your dissertation (maybe faster!).

This bacterial system is called CRISPR/Cas. Bacteria use it to defend against the foreign DNA—from infecting phages, bacteria that transfer plasmids via conjugation, or other sources—that constantly assaults them. And now scientists are using it to edit the genomes of most any beast they want, including our favorite yeast.

What makes this technique so powerful is that it is easily programmable. You can make virtually any sequence change that you wish, and even more impressively, make lots of changes all at once. CRISPR/Cas may be one of those technological leaps that changes everything.

The incredible potential of this approach, and the ethics of its use in humans, are hot topics right now. But while researchers and philosophers are hammering out guidelines for using the CRISPR/Cas system in larger organisms, yeast researchers are free to forge ahead and edit the S. cerevisiae genome to their hearts’ content. This has now been made much easier with the availability of a whole toolkit, created at the Delft University of Technology and described in a new paper by Mans et al. 

The yeast system has three essential components. The first is a plasmid that will express a specific guide RNA (gRNA). The gRNA leads a nuclease to the right place in the genome.  Mans and colleagues made a whole set of plasmids, with different nutritional and antibiotic resistance markers, that can express one or more gRNAs.

The second component of the system is the nuclease that binds to the gRNA and makes a double-stranded cut in the target DNA. The researchers used the Cas9 nuclease from Streptococcus pyogenes, and engineered a set of yeast strains that had the cas9 gene stably integrated into a chromosome and expressed from a strong yeast promoter.

The third component is one or more repair fragments: pieces of DNA that specify the modified sequence that the researcher wants to engineer into the genome.

So, for example, if a scientist wants to delete a gene precisely, she can create a gRNA that targets the gene, and then co-transform a Cas9-expressing yeast strain with both the plasmid expressing the gRNA and a repair fragment that corresponds to the gene’s upstream and downstream flanking sequences, fused together. When the gRNA is expressed in the transformant, it leads Cas9 to the gene, where it makes a double-stranded cut at a precise position in the DNA.

Now the researcher can let yeast do the rest of the work. S. cerevisiae has a powerful homologous recombination system, and it’s greatly stimulated by double-stranded breaks in DNA.

After Cas9 cuts the gene, yeast will repair the break, using as a template the repair fragment that the researcher designed. In this case, the upstream and downstream sequences will recombine with the homologous sequences in the chromosome, but since the coding sequence is missing from the repair fragment, the resulting strain will have a precise deletion of the gene of interest.

This example illustrates one of the simplest uses of the technique. Mans and colleagues tried successively more complicated tasks and were able to accomplish some amazing feats.

They were able to precisely delete six genes in one step by transforming with repair fragments for all six, along with three plasmids that each expressed two gRNAs. Also in one step, they replaced one yeast gene with six Enterococcus faecalis genes encoding subunits of the pyruvate dehydrogenase complex and other enzymes in the pathway. The E. faecalis genes were specified on six overlapping repair fragments that were cotransformed into the strain.

The CRISPR/Cas9 system designed by Mans and colleagues can do much more than gene deletions and replacements. By designing repair fragments specifying particular mutations, the method can also be used to create point mutations or other modifications.

For this technique to work, it’s important that there are no mismatches between the gRNA sequence and the chromosomal target sequence, and other sequence characteristics can influence the efficiency of the method.  So the authors created an online tool that helps researchers select optimal Cas9 targets in regions of interest and design gRNA sequences. Since they incorporated the sequences of 33 different S. cerevisiae strains into the tool, researchers can specify a strain and retrieve information on the best sequences for targets and gRNAs for their gene(s) of interest based on sequences found in that particular strain.

Importantly, the CRISPR/Cas9 technique allows researchers to make multiple changes in a single step. This is a big advantage, since transformation itself can be mutagenic. For example, a strain that has been commonly used to investigate the function of hexose transporters was engineered to carry multiple deletions in the conventional manner, using successive rounds of transformation and selection.  Its genomic sequence, which was recently determined, reveals that its genome is a complete mess, with many rearrangements and deletions. 

With the development of this toolkit, editing the S. cerevisiae genome is beginning to be almost as easy as editing a text document. And since Mans and colleagues have made all of the strains, plasmids, and online tool freely available to the world, everyone will be able to take advantage of them. Just think of the stories that yeast researchers will be able to write!

CRISPR/Cas in Bacteria

Way before this became a powerful tool for researchers, it was a very cool immune system for bacteria, allowing them to defend against assaults from foreign invaders. 

Bacteria collect foreign DNA sequences from invaders that threaten them, just like collecting mug shots of notorious criminals. They store these mug shots in a special place in their genome, integrated between blocks of a repeated sequence. (The CRISPR acronym refers to these repeats.) This region is transcribed, and the RNA is chopped into pieces containing individual mug shots. Because these mug shots, called crRNAs, are complementary to the DNA sequences of invaders, they can recognize and hybridize with those invading sequences.

Bacteria have an additional small RNA, the tacrRNA, that binds to both the crRNA and to a CRISPR-associated (Cas) nuclease. This forms a RNA-DNA-protein complex on the foreign DNA and allows the nuclease to do its work, cutting both strands of the DNA and neutralizing the invader.

To use this system for genome engineering, scientists have fused the two RNAs of the bacterial system into a single RNA, the guide RNA (gRNA).  It contains both the mug shot (with sequences complementary to the target) and the RNA sequence that recruits the nuclease. The gRNA, a Cas nuclease, and a repair fragment are the essential components of the system.

by Maria Costanzo, Ph.D., Senior Biocurator, SGD

Categories: Research Spotlight

Tags: synthetic biology, Saccharomyces cerevisiae, genome engineering, CRISPR/Cas

From Sourdough Bread to Chemotherapeutic Drugs

March 18, 2015


Microbes can achieve great things when they work together:

Microbiologists in the lab spend a lot of time and effort keeping each microbial strain and species separate. The conventional wisdom is that if you want to really understand an organism, you need to study it in isolation, as a pure culture. If contaminating colonies of some other bug appear on your Petri dishes, you’d better melt down those plates in the autoclave and trash them!

When microbes work together, the results range from delicious to life-saving. Image by Hillarywebb via Wikimedia Commons

On the other hand, amateur microbiologists have known for centuries that mixtures of microbes can do great things. Sourdough bread, for example, is made using a culture of Lactobacilli and yeasts. They complement each other during the fermentation: the bacteria metabolize sugars that the yeast can’t use, and make new compounds that can be fermented by the yeast. The result is extremely tasty. 

A new study from Zhou and colleagues brings a microbial community into the lab to make a medicine called paclitaxel that is used to treat cancer. Yes, bread bowls for your clam chowder are cool, but this is obviously way more important for human health.

Paclitaxel is an incredibly successful drug for treating breast and ovarian cancer, but unfortunately there isn’t an easy way to make it.  It can be purified from the bark of the Pacific yew (killing the trees in the process), synthesized by plant cells cultured in vitro, or synthesized chemically. But all of these processes are expensive and complicated, which means this life saving drug is always in short supply.

The researchers wanted to produce it more cheaply and easily, and an obvious solution was to let microbes do most of the work. But neither of the most commonly used microbial workhorses, S. cerevisiae and E. coli, was exactly right for the job.

E. coli had already been engineered to overproduce the compound taxadiene. The taxadiene then needs to be oxidized to create oxygenated taxanes, which are paclitaxel precursors. This oxidation can be done by membrane-bound oxidoreductase enzymes called cytochrome P450s. But these enzymes are not found naturally in bacteria, and getting them expressed and functional in E. coli is challenging.

The researchers decided to see whether they could coax these two microbes into cooperating to produce oxygenated taxanes. After creating an E. coli strain that produced taxadiene and an S. cerevisiae strain that produced a P450 oxidoreductase, they grew them together in the same culture, with glucose as the carbon source. 

As planned, the E. coli pumped out taxadiene and it was able to diffuse into the yeast cells, where it became oxygenated. However, the two species weren’t as happy together as the researchers had hoped. 

One of the things that humans love about yeast is that when it grows on glucose, it produces ethanol. However, the E. coli cells didn’t love being bathed in ethanol: their yield of taxadiene went way down as the ethanol levels in the culture went up.

So Zhou and coworkers switched the carbon source to xylose. S. cerevisiae cannot consume xylose, but E. coli can. When growing on xylose, E. coli produces acetate, which the yeast can use—and they don’t produce ethanol under these conditions.

Growing the microbes in xylose doubled the yield of oxygenated taxanes over that of the glucose-grown culture. But still, only 8% of the taxadiene that was produced was getting oxygenated.

To be sure that the yeast cells were producing the P450 enzyme as efficiently as possible, the researchers tried driving transcription of the gene using several different promoters. Using the promoter that was strongest in the co-culture conditions made a significant difference in the proportion of taxadiene that was oxygenated.

The researchers guessed that another factor limiting the final yield was that the yeast cells were not growing as well as they could. Tweaking the ratio of the two species in culture and the contents of the media resulted in a three-fold increase in oxygenated taxanes. But Zhou and colleagues hoped to improve things even more.

Thinking that the limiting step in yeast growth might be the supply of acetate, the researchers tried to beef up the acetate synthesis pathway in E. coli. They engineered the bacteria to overproduce several of the enzymes in the acetate biosynthesis pathway, but this didn’t make a large difference. 

Scientists came up with expensive ways to stop using the Pacific yew tree to make paclitaxel. Now we might be able to do this more cheaply with yeast and bacteria. Image by Jason Hollinger via Wikimedia Commons

They reasoned that if they forced the E. coli to rely on the acetate biosynthesis pathway for energy, the cells might ramp up their acetate production. To do this they blocked oxidative phosphorylation by deleting the the atpFH gene that encodes a subunit of ATP synthase. Now more acetate was produced, the yeast cells grew better, and 75% of the taxadiene that was produced got oxygenated. They were in business!

Zhou and coworkers went on to show that the co-culture environment could be modified to generate several other isoprenoids. This class of naturally-occurring molecules includes some that are in use, or in development, as pharmaceuticals (paclitaxel, and others currently in clinical trials) and compounds that have other applications, from fragrances to fuels. 

There’s much more work to be done, and the potential of microbial communities is just beginning to be realized. Harnessing the power of multiple organisms means that different steps of pathways can be optimized separately and then mixed and matched for a desired result. This approach could turn out to be the best thing since sliced bread! But then again, sourdough bakers already knew that.

by Maria Costanzo, Ph.D., Senior Biocurator, SGD

Categories: Research Spotlight

Tags: paclitaxel, Saccharomyces cerevisiae, synthetic biology

Telomerator is the Chromosomal Terminator

December 10, 2014


A simple tool for adding telomeres to linear DNA:

First there was the Terminator. Now, in a new study published in PNAS by Mitchell and Boeke, we have the telomerator.

When he cuts you up, you won’t survive. But when the telomerator cuts your circular DNA, it will survive fine as a stable, linearized piece of DNA. Image via Wikimedia Commons

Instead of being a homicidal robot from the future bent on killing Sarah Connor, the telomerator is a tool that lets scientists easily turn circular DNA into stable chromosomes in the yeast Saccharomyces cerevisiae. While less splashy, this bit of synthetic biology is definitely cool in its own way (and much less dangerous!).

The system that Mitchell and Boeke created is very clever. They first inserted the intron from the ACT1 gene into the middle of the URA3 gene. The URA3 gene was still functional, as ura3 mutants could use it to grow on medium lacking uracil.  

They next inserted a sequence into the middle of the intron that consisted of an 18 base pair I-SceI cleavage site flanked on each side by around 40 base pairs of yeast telomere repeats (called Telomere Seed Sequences or TeSSs). This construct still allowed ura3 mutants to grow in the absence of added uracil.

The final step was to introduce the homing endonuclease I-SceI to the cell so that it cut the circular DNA precisely between the two TeSSs. The idea is that when you add the homing endonuclease, the newly linearized piece of DNA ends up with the telomere seeds on each end. Telomerase adds more repeats to the seeds until the DNA has proper telomeres. Voilà, a chromosome is born.

The URA3 gene part of the plasmid is important for selecting cells with the linearized DNA. Basically a circularized DNA will grow on medium lacking uracil but fail to grow on medium with 5-FOA, while the linearized DNA will do the opposite. In other words, the process of linearization should destroy the URA3 gene. And that’s just what they found.

Previous work had shown that to be stable in yeast, a chromosome needs to be at least 90 kilobases (kb) or so long. This is why they tested their new telomerator in synIXR, a synthetic yeast chromosome that is about 100 kb in length. This chromosome has 52 genes from the right arm of chromosome 9, two genes from the left arm, around 10 kb of nonessential BAC DNA, the native centromere CEN9, and a LEU2 marker. 

Mitchell and Boeke inserted the telomerator sequence into two different locations in the BAC part of the circularized synIXR and found that adding I-SceI appeared to linearize the DNA. In both cases they found that around 100 out of 200 cells were resistant to 5-FOA and unable to grow in the absence of uracil but could still grow in the absence of leucine. This is just what we would predict if we cut the DNA in the middle of the URA3 gene and created a stable piece of linear DNA.

They next wanted to use this tool to study the effects of telomeric DNA on nearby genes. We would predict that because of telomeric silencing, genes near a telomere will be downregulated. Any genes that affect growth when turned down should quickly become evident.

To accomplish this they inserted the telomerator three base pairs downstream of each of the 54 genes on synIXR, generating 54 new plasmids. After activating the telomerator by expressing the I-SceI nuclease, they used pulsed field gel electrophoresis to confirm that 51 of the 54 synthetic chromosomes had indeed been linearized.

As expected, they found that putting a telomere near a gene sometimes has profound effects. For example, when they linearized DNA where the telomerator was 3’ of either YIR014W, MRS1, or YIR020C-B, they got no growth. They also found many more effects on the growth rate at both 30° C and 37° C at many different, “telomerized” genes. The implication is that when these genes are near telomeric DNA, they no longer function at a high enough level for the yeast to grow well or in some cases to even survive.

To confirm that the effects they saw were due to telomeric silencing, Mitchell and Boeke tested each linearized DNA in a sir2 mutant, a key player in this form of silencing. Mutating sir2 reversed the effects of placing a telomere near the gene, further supporting the idea that the newly created chromosome ends are like normal telomeres because they undergo the same Sir2-mediated silencing.

Finally, the researchers tested the stability of the newly created chromosomes by selecting for Ura+ revertants from six individual cultures with different linearized molecules. They failed to select any revertants in which the DNA had recircularized, showing that the linear chromosomes are stable.

So in contrast to the Terminator, who sliced and diced his victims randomly, the telomerator will allow synthetic biologists to create linear chromosomes with precisely positioned telomeres. This study proved the concept, and this tool will be incredibly useful in the future, both in yeast and potentially in other eukaryotes. Both the Terminator and the telomerator can say, “I’ll be back”!

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: synthetic biology, Saccharomyces cerevisiae, telomere

Adding Introns to Synthetic Biology’s Toolbox

July 03, 2014

As any good handyman knows, the more tools you have in your tool chest, the better the chance that you can find what you need to solve a problem.  The same goes for synthetic biologists.  The more parts they can mix and match, the more likely they are to engineer the exact level of gene expression they need.

Synthetic biologists have added introns to their tool chest. Image from Wikimedia Commons

In the last few years synthetic biologists have amassed a wide variety of transcription and translation elements that can be combined in different ways to exquisitely tune the level of expression of their gene of interest.  And now, in a new study out in PLOS Genetics, Yofe and coworkers have added introns to the list of parts available for our favorite yeast Saccharomyces cerevisiae.

Yeast isn’t loaded with introns, but it does have a reasonable number that can be co-opted for synthetic biology.  The authors inserted 240 of these introns individually into the same position near the 5’ end of the yellow fluorescent protein (YFP) gene and monitored the level of fluorescence of each individual strain over a 24 hour period.  They chose the 5’ end of the gene because yeast has a bias for introns being located there.

The authors found that these reporters spanned a 100-fold range of gene expression, that every intron caused a decrease in the level of gene expression, and that even though many of these introns respond to environmental stimuli in their natural context, their effect on gene expression here was immune to the environmental changes the authors tested.  Taken together, these results suggest that introns could be used in yeast systems for dampening over-exuberant gene expression in ways that are independent of growth conditions.  If all of this holds up, introns will prove to be very useful tools indeed.

Yofe and coworkers next wanted to use this library to figure out some of the rules for why some introns cause lowered activity compared to others.  The simplest possibility, that longer introns cause a larger decrease in gene expression, turned out not to be true.  There was no correlation between the size of the intron and its effect on the level of fluorescence. 

Next they scanned the sequences of their constructs to look for elements that might increase or decrease splicing efficiency.  These splicing regulatory elements (SREs) are better understood in larger eukaryotes, but there is evidence that they are important in yeast as well.  The authors identified a number of intron splicing enhancers (ISEs) and intron splicing silencers (ISSs) that were highly enriched near the splice sites. 

To confirm that these sequences did in fact affect splicing efficiency (and hence gene expression), they showed that mutating the enhancer motif TTTATGCT to the silencer motif TTTGTGTA in two reporters resulted in a 22% and a 13% decrease in gene expression.  This proof of principle experiment suggests that future synthetic biologists may be able to further tweak the expression of their genes by manipulating these SREs.

In a final set of experiments the authors used the library to identify rules that can be used to predict how inserting various introns into different positions will affect a gene’s activity.  They found that the most important features were the presence of SREs and the RNA structures at the intron-exon junction.  Synthetic biologists should be able to use these rules to intelligently design their reporter systems.

These experiments are the first step towards adding introns to the ever growing set of tools available to synthetic biologists for modulating gene expression.  We are getting closer to figuring out how genes are controlled and being able to use that knowledge to our advantage.  Or to put it another way, we have taken another baby step towards being able to control a gene as well as a yeast cell does.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: splicing, Saccharomyces cerevisiae, synthetic biology

Redesigning Yeast One Chromosome at a Time

April 03, 2014

Everyone who reads our blog knows how awesome the yeast Saccharomyces cerevisiae is.  Without this little workhorse we would almost certainly not understand ourselves as well as we do now.  It is an indispensable tool in figuring out how eukaryotes work.

Scientists have taken the first step in making yeast an even better all purpose tool than it already was. Image from Wikimedia Commons

And of course yeast is much more than that.  It makes our bread fluffy and our drinks alcoholic.  It can be manipulated into making medicines like artemisinin, a powerful anti-malarial drug, or biofuels or whatever else we can think of.  It is the Swiss Army Knife of useful organisms.

Even with all of this fanfare, everyone knows yeast has its limitations.  It is a powerful tool but it could be improved.  For example, it would be nice if researchers could more easily manipulate its DNA to speed up the introduction of beneficial traits, add new biosynthetic pathways, or to do the kinds of experiments that will help one day cure cancer or Alzheimer’s disease.  This is where Sc2.0 comes in.

Sc2.0 is an idea that has been kicking around for the last decade or so.  First proposed by Ron Davis of Stanford University, the idea is to synthesize artificial yeast chromosomes to make yeast more useful.  Eventually the idea would be to recreate every yeast chromosome and intelligently redesign the genome for our own purposes. And maybe even to add new artificial chromosomes so we can easily add whatever genes we want.

In a new study out in Science, Annaluru and coworkers have taken a major step forward in the Sc2.0 project by replacing all 316,617 base pairs of yeast chromosome III with a 272,871 base pair synthetic version, synIII.   That leaves only 15 chromosomes and around 12.2 million base pairs before we have yeast with completely manmade DNA.

Annaluru and coworkers managed to do this with the help of a bunch of undergraduate students and yeast’s love of homologous recombination.  The first step was to have undergraduates synthesize around 30,000 base pairs each in the “Build a Genome” class at Johns Hopkins.  It took 49 students around 18 months to pull this off for synIII.

Basically they used 60-mer and 79-mer oligonucleotides to PCR up 750 base pair building blocks.  These pieces of DNA were designed so that they could be assembled into 2,000-4,000 base pair minichunks.  The final step was to transform yeast with an average of twelve of these minichunks and to let the yeast use homologous recombination to replace its native DNA sequence with the added DNA.  After 11 rounds of transformation, the yeast now had an artificial chromosome.

As you may have guessed, this chromosome is not exactly the same as the one it replaced.  To eventually free up a codon for repurposing later, all 43 of the TAG stop codons were converted to TAA.  When this is done with all of the chromosomes, researchers will now have a codon they can use to change this yeast’s fundamental genetic code.  This might allow for adding novel amino acids to proteins or even prevent viruses from infecting the new yeast.

Annaluru and coworkers also introduced 98 loxP sites which in the presence of estradiol will cause the yeast to undergo rapid DNA change.  The hope is that scientists will be able to harness SCRaMbLE (synthetic chromosome rearrangement and modification by loxP-mediated evolution), as it has been named, to more quickly evolve useful traits in yeast for both study and biotechnological uses.

As a final step, the researchers cleaned up the chromosome by removing 21 retrotransposons and many introns and by moving 11 tRNA genes to a neochromosome.  They now had created a leaner, meaner chromosome III. 

The next obvious question was whether or not all of these changes affected the yeast.  Despite looking very carefully, Annaluru and coworkers could find little that was different between strains carrying natural and synthetic chromosomes.  They both grew similarly under 21 different conditions in terms of growth curves, colony size, and cell morphology, and had very similar transcription profiles.  But they weren’t identical.

For example, the strain with synIII grew slightly less well in the presence of high sorbitol, and showed differences in expression from wild type in 10 out 6,756 transcripts.  Of these ten, eight were intentionally altered in the creation of synIII and so were expected.  The two unexpected changes were a ~16-fold decrease in the expression of HSP30 on synIII and a ~16-fold increase in the expression of PCL1 on chromosome XIV.

Since all of these changes had such a small effect on the yeast, it is a green light for plowing ahead with creating yeast with completely manmade DNA.  Currently four other chromosomes, II, V, VI, and XII, are nearly done and the design work has been completed for chromosomes I, IV, VII, and XI (see an overview of the project).  It will only be a matter of time before we have a strain of yeast with completely synthetic DNA.  Scientists are making a powerful tool even better…who knows what this new strain will help us discover.

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

Categories: Research Spotlight

Tags: synthetic biology, Saccharomyces cerevisiae, teaching

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