New & Noteworthy

Yeast Tackles Climate Change

November 29, 2016

With its new superpower of turning xylose into ethanol, yeast is just about ready to provide the low carbon fuel we need to help us stave off climate change. Image from pixabay.

One of the best parts about doing outreach with a museum is creating a successful hands on activity for the visitors. This is not an easy thing to do.

You first create something that you think will appeal to and educate visitors. When that falls flat on its face, you then do a series of tweaks until it is working smoothly. In the end you have smiling kids who understand DNA better (or at all)!

Genetic engineering can be similar. You can import the genes for a complex pathway into your beast of choice but it may not work first try. A bit of tinkering, evolution style, is often needed to get the engineering working well enough to be useful.

This is exactly what happened when a group of researchers tried to get our favorite beast, the yeast Saccharomyces cerevisiae, to turn the sugar xylose into ethanol. They added the right genes from either fungi or bacteria, but S. cerevisiae couldn’t convert enough xylose into ethanol to be useful.

And it is important for all of us that some beast be able to do this well. A yeast that can turn xylose into ethanol means a yeast that can turn a higher percent of agricultural waste into a biofuel. Which, of course, means lots of low carbon fuel to run our cars so that we have a better shot of limiting the Earth’s heating up by 1.5-2 degrees Celsius.

To get their engineered yeast to better utilize xylose, Sato and coworkers forced it to grow with xylose as its only carbon source. Ten months and hundreds of generations later, this yeast had evolved into two new strains that were much better at turning xylose into ethanol. One strain did its magic with oxygen, the other without it.

In a new study in PLOS Genetics, Sato and coworkers set out to figure out which of the mutations that came up in their evolution experiments mattered and why.

Two genes were common in both the aerobic and anaerobic strains – HOG1 and ISU1. Both needed to be nonfunctional in order to maximize ethanol yields from xylose. They confirmed this by deleting each individually and together from the parental strain.

HOG1 encodes a MAP kinase, and ISU1 encodes a mitochondrial iron-sulfur cluster chaperone. These probably would not have been the first genes to go after with a more biased approach. The benefits of evolution and natural selection!

Further experiments showed deleting each gene individually was not as good as deleting both at once when oxygen was around. In fact, while deleting only ISU1 had a small effect on the ability of this yeast to convert xylose into ethanol, deleting HOG1 alone had no effect at all. Its deletion can only help a strain already deleted for ISU1.

In the absence of oxygen, yeast needs a couple of additional genes mutated – GRE3 and IRA2. GRE3 is an aldose reductase and IRA2 is an inhibitor of RAS. Again, not very obvious genes!

Still, once you find the genes you can come up with reasonable hypotheses for why they are important.

Some are easier than others. Hog1p, for example, is known to enhance a cell’s ability to turn xylose into xylitol, which shunts the xylose away from the ethanol conversion pathway. GRE3 is involved in this as well. Deleting either should make more xylose available to the yeast.

This doesn’t mean this is Hog1p’s only role in boosting this yeast’s ability to turn xylose into ethanol of course. It also probably “…relieves growth inhibition and restores glycolytic activity in response to non-glucose carbon sources.” Consistent with this, the authors found that the xylose-utilizing strain deleted for HOG1 was also better at using glycerol and acetate as carbon sources.


The tweaks needed to make a successful museum activity are often nonintuitive. Much like the tweaks needed to maximize the performance of genetically engineered yeast. Image courtesy of The Tech Museum.

Other genes were less obvious. For example, perhaps mutating ISU1 frees up some iron so extra heme can be made. Or alternatively, it may have increased the mass of mitochondria available. Again, probably would not have been the first gene to go after to improve yeast’s ability to convert xylose to ethanol.

Which again underlines the importance of letting natural selection improve an engineered organism as opposed to only trying to pick and tweak the genes you think are important. Biology is simply too complicated and our understanding too limited to be able to know which are the best genes to go after. This is reminiscent of prototyping museum activities.

Some tweaks are obvious but others you would never have guessed would be needed. For example, we had visitors spreading bacteria on a plate and found that if they labeled their plate first, they almost always put their transformation mixture on the lid instead of on the LB agar. This problem was solved by having them add their mixture first and then labeling their plate.

It would be very hard to predict something like this from the get-go. The activity needed to evolve on the museum floor to work optimally. Much like the yeast engineered to utilize xylose needed to evolve in the presence of xylose to work optimally. And to perhaps take a big step towards saving the Earth from warming up too much.

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

Categories: Research Spotlight

Tags: biofuel, ethanol, evolution, fermentation, glucose metabolism, xylose

A Factory Without Doors

April 29, 2015

Every factory needs raw materials. Without steel, this is just a pretty factory. And without incoming xylose, a yeast cell set up to make ethanol from biomass is just a pretty cell. Image by Steve Jurvetson via Flickr

Imagine you have built a state-of-the-art factory to make a revolutionary product. The place is filled with gleaming assembly lines and you have hired the best talent in the world to run the place.

Unfortunately there was a glitch in the factory design—the builders forgot to put doors in! Now you can’t get the raw materials in to make that killer product that will change everything.

This may sound contrived or even silly, but it is sort of what is happening in attempts to use yeast to make biofuels from agricultural waste. Scientists have tweaked yeast cells to be able to turn xylose, a major sugar found in agricultural waste, into ethanol. But yeast has no transporter system for this sugar. A bit can get in through the windows, so to speak, but we need to put in a door so enough can get inside to make yeast a viable source for xylose-derived ethanol.

An important step was taken in this direction in a new study by Reznicek and coworkers. They used directed evolution to transform the Gal2 transporter of Saccharomyces cerevisiae into a better xylose transporter. And they succeeded.

After three successive rounds of mutagenesis, they transformed Gal2 from a transporter that prefers glucose into one that prefers xylose. When put in the right background, this mutant protein opens the door for getting yeast to turn agricultural waste into ethanol. Perhaps yeast can help us stave off cataclysmic climate change for just a bit longer. 

The first step was to find the right strain for assaying xylose utilization. They needed a strain lacking 8 hexose transporters, Hxt1-7 and Gal2, because these transporters can take up xylose (albeit at a very low efficiency). Deleting these genes “shuts the windows” and completely prevents the strain from utilizing xylose as a substrate (as well as impairing its ability to use glucose).

This strain was also engineered to be able to utilize xylose. It contained a xylose isomerase gene from an anaerobic fungus and also either overexpressed or lacked several S. cerevisiae genes involved in carbohydrate metabolism. With this strain in hand, the researchers were now ready to add a door to their closed off factory.  

The authors targeted amino acids 292 to 477 in Gal2. This region is thought to be critical for recognizing sugars, based on homology with other hexose transporters. They used mutagenic PCR conditions that generated an average of 4 point mutations in this region, and screened for mutants that grew better than others on plates containing 0.1% xylose.

In their initial screen they selected and replated the 80 colonies that grew best. They then chose the best 9 to analyze further. Of these 9, one mutant which they dubbed variant 1.1 grew better on xylose than a strain carrying wild-type GAL2. Variant 1.1 had a single amino acid change, L311R.

They repeated their assay using variant 1.1 as their starting source. Out of the 14,400 mutants assayed, they found four that did better than variant 1.1. These variants, dubbed 2.1-2.4, all shared the same M435T mutation.  Variant 2.1 had three additional mutations—L301R, K310R, and N314D.

These four new mutants showed better growth on 0.45% xylose, and after 62 hours, all the strains had pretty much used up the xylose in their media. Of the four, variant 2.1 appeared to be the best xylose utilizer: after 62 hours the authors could detect no xylose in the media at all. This variant also grew faster than the others in 0.1% xylose.

Reznicek and coworkers had definitely made Gal2 a better xylose transporter, but they weren’t done yet. They wanted to try to make a door that only let in the raw supplies (xylose) they wanted and not other sugars (glucose).

Up until now, the screens had been done with xylose as the sole carbon source. When they grew variant 2.1 in the presence of both 2% glucose and 2% xylose, they found that it preferentially used the glucose first. Their evolved transporter still preferred glucose over xylose!

Now in some ways this wasn’t surprising, as the mutations had not really affected the part of the protein thought to be involved in recognizing sugars. They next set out to evolve Gal2 so that it would transport xylose preferentially over glucose.

This time they used a slightly different background strain for their screen. This strain, which was deleted for hxk1, hxk2, glk1, and gal1, was unable to use glucose although it could transport it.

They repeated their mutagenesis and looked for mutants that grew best in 10% glucose and 2% xylose. We would predict that any growing mutants would have to transport xylose better than glucose. And this is just what they found.

When they analyzed the mutants, they found that the key mutation in making Gal2 prefer xylose over glucose in the variant 2.1 background was T386A. Based on homology with Hxt7, this mutation happens smack dab in the middle of the sugar recognition part of the protein. Most likely this mutation compromised the ability of Gal2 to recognize glucose, as opposed to improving recognition for xylose.

These experiments represent an important but by no means final step in engineering yeast to make fuel from biomass. We are on our way to a smaller carbon footprint and perhaps a world made somewhat safer from climate change.

First, beer, wine, and bread; next, keeping coral alive and saving countless species from extinction. Nice work, yeast.

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

Categories: Research Spotlight

Tags: biofuel, evolution, Saccharomyces cerevisiae

If Yeast Can’t Stand the Heat, Our World May Be in Trouble

October 23, 2014

In case there were still any doubters, the world is definitely heating up. April to September 2014 were the warmest these months have ever been since we started keeping records in 1880. And about 35,000 walruses were forced ashore this summer in Alaska because there wasn’t enough room left for them on the sea ice where they normally hang out. Unfortunately, the trend looks to be more record breaking heat for as long as we can see in the future.

Like the X-men, mutated yeast might one day be the superheroes that save us from the worst effects of global warming. Image by Gage Skidmore via Wikimedia Commons

This is bad news for us, and even worse news for nature.  But all is not lost! Our hero yeast may be able to swoop in and make us biofuels to replace gasoline.  This won’t stop global warming, but it might at the very least slow it down.

But yeast in its current form probably couldn’t make enough biofuels to make a difference without using too much precious food in the process. Like the X-men, it will need a helpful mutation or two to increase its efficiency to the point where it can make a real dent in slowing down the relentless rise in global temperatures. And ironically, one new trait yeast needs to pull this off is to be able to survive better in the heat.

To efficiently turn the parts of the plants we can’t use (mostly anything to do with cellulose) into biofuels, we need for yeast to grow in cultures where enzymes are chopping cellulose into bite sized pieces at the same time.  These enzymes work best at temperatures that are a real struggle for wild type yeast.

In a new study in Science, Caspeta and coworkers isolated mutants better able to tolerate high temperatures by simply growing Saccharomyces cerevisiae at around 39.5° C for over 300 generations.  They did this in triplicate and ended up with strains that grew on average about 1.9 times faster than wild type yeast at these temperatures.

The next step was to use genome sequencing and whole-genome transcription profiling to figure out why these mutant strains were heat tolerant. This is where things got really interesting.

They found many genes that were affected, but identified ERG3 as the most important player. All of the thermotolerant strains that they selected contained a nonsense mutation in ERG3. And in fact, when they introduced an erg3 nonsense mutation into wild type yeast, they found that this engineered strain grew 86% as well as the original mutant strain at high temperatures. In other words, most of the heat tolerance of these strains came from mutations in the ERG3 gene.

This makes sense, as membrane fluidity is a key factor in dealing with higher temperatures, and ERG3 codes for a C-5 sterol desaturase important for membrane composition.  When Caspeta and coworkers looked at the membranes of the mutant strains, they found a buildup of the “bended” sterol fecosterol.  Since “bended” sterols have been shown to protect the membranes of plants and Archaea from temperature swings, this could be the reason that the mutant strains dealt with the heat so well.

So as we might predict, changing the composition of the cell membrane affects how the yeast respond to temperature. What we couldn’t predict is that in order to get to this point via artificial selection, the yeast had to have a second mutation in either the ATP2 or the ATP3 genes that actually make yeast less able to grow at higher temperatures.

Because ATP2 and ATP3 are needed for yeast to grow on nonfermentable carbon sources, the authors hypothesized that perhaps thermotolerance could not evolve while oxidative respiration worked at full speed. Consistent with this, Caspeta and coworkers found that their evolved thermotolerant strains were more susceptible to oxidative stress and could not grow on nonfermentable carbon sources. This was not true of the strain they engineered to only have the mutation in the ERG3 gene—it grew well at 40° C and could use nonfermentable carbon sources.

So not only have these authors found a mutant yeast with the superpower of growing at higher temperatures, but they also showed that sometimes engineering works better than natural selection at creating the mutant they want. Evolving sometimes requires passing through an intermediate that makes the final product less useful than it could have been. In this case it worked best to use a combination of artificial selection and engineering to build a better mutant.   

Even this super engineered yeast mutant won’t stop global warming in its tracks, but it might help us to slow it down enough so that natural systems have a chance to adapt. A superhero indeed… 

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

Categories: Research Spotlight

Tags: biofuel, evolution, Saccharomyces cerevisiae, thermotolerance

Have Your Fuel and Eat It Too

September 19, 2013

Back in 2008 and 2011 there were huge spikes in the cost of food that caused riots in various parts of the world.  These things were pretty bad and one of our favorite beast’s best products, ethanol, may have been at least partly to blame.  In an attempt to deal with global warming, governments had created incentives that made it more lucrative to turn food into ethanol to power cars rather than keeping it as food to feed people.  The law of unintended consequences reared its ugly head and caused food prices to rise high enough to be unaffordable by the very poor.   

Getting yeast to turn more of this into ethanol is good for us and the environment.

This situation arose because right now, pretty much the only commercially viable way to make ethanol is to use sugars like those found in sugar cane or starches like those found in corn.  Ultimately this won’t be a problem once scientists learn to coax yeast or other microorganisms to make ethanol out of agricultural waste.  Until then, though, one way to lessen the impact of ethanol production on food supplies might be to engineer a yeast strain that can more efficiently turn sugars into ethanol. 

One of the most inefficient parts of yeast fermentation is that the silly thing converts anywhere from 4-10% of the sugars it gets into glycerol instead of ethanol.   In a new study, Guadalupe-Medina and coworkers have engineered a strain of yeast that produces 60% less glycerol and 8% more ethanol than other commercial strains.  If they can scale this up, it might help us feed both the world’s population and our cars.

It has been known for some time that yeast end up making glycerol during fermentation because of redox-cofactor balancing issues.  In essence, the excess NADH that is made in fermentation reactions is reoxidized by converting part of the sugar into glycerol.  One obvious way to get less glycerol would be to give the yeast some other way to reoxidize its NADH. 

Guadalupe-Medina and coworkers decided to persuade yeast to use carbon dioxide instead of sugars.  Not only would this make sugar use more efficient, but their particular plan would also convert that carbon dioxide into a precursor that could be shunted into the ethanol producing pathway.  Theoretically the yeast should now increase its ethanol production both by wasting less sugar on glycerol and by turning carbon dioxide into ethanol.  And it turns out that this idea actually worked in practice.

The first step was to introduce the Rubisco enzyme into the yeast.  Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase) is really one of the key enzymes in life…it provides the foundation for almost all life on the planet by fixing carbon dioxide from the air into ribulose-1,5 phosphate.  But that isn’t the important point here.  No, the key point for this work is that in the process of doing this, the enzyme oxidizes NADH.  By putting Rubisco in yeast, the yeast should now be able to reoxidize its NADH without making useless glycerol.

Of course this is easier said than done!  Rubisco is multi-subunit in most beasts and persnickety to boot.  But with a bit of work, they managed to get Saccharomyces cerevisiae to express a working copy of Rubisco.

So they would only have to introduce a single gene, the authors used the single subunit enzyme from T. dentrificans. As expected, this gene alone was not enough.  They knew from previous work that Rubisco would not work in yeast without the help of a couple of E. coli chaperones, groEL and groES.  When they expressed all three genes at the same time, they got Rubisco to fix carbon dioxide in Saccharomyces cerevisiae.  

The next step was to introduce the enzyme phosphoribulokinase (PRK) so that the ribulose-1,5 phosphate could be converted into 3-phosphoglycerate, a precursor in the ethanol pathway.  Luckily this was much easier than Rubisco and worked on the first try.  They had now engineered a Frankenyeast that should be able to make more ethanol and less glycerol.

When they tested the new strain, Guadalupe-Medina and coworkers found they had indeed engineered a more efficient yeast.  In anaerobic chemostat conditions, this yeast made 68% less glycerol and 11% more ethanol than the usual commercial strain.  They obtained similar results, 60% less glycerol and 8% more ethanol, in batch fermentations.  They had succeeded in improving an already awesome beast.

If this strain works on an industrial scale and if commercial producers all used this strain instead of the ones they currently use, the authors calculate we could get an extra 5 billion liters of ethanol added to the 110 billion we are already making.  That might just be enough to tide us over until scientists come up with a way to make ethanol commercially from non-food sources.

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

Categories: Research Spotlight

Tags: biofuel, Saccharomyces cerevisiae

Saccharomyces cerevisiae, the Party Animal

June 26, 2013

S. cerevisiae would be the life of a fraternity party. With its high alcohol tolerance, it could win every beer drinking contest. And if the party ran out of alcohol, it could make lots more!

Like a barfly before his liver gives out, the yeast S. cerevisiae can tolerate incredibly high levels of ethanol.  But unlike the town drunk, S. cerevisiae uses this skill to its own advantage.

In the wild, this yeast ferments sugars to flood the local environment with alcohol.  The end result is that it does just fine but any nearby microorganisms are killed.

Humans also take advantage of this unique property of S. cerevisiae.  Not only does it allow us to brew beer and ferment grapes into wine without worrying too much about bacterial contamination, but it also helps us generate ethanol as a biofuel.  What a cool and useful little beast!

Given how important this property is for both yeast and us, it is perhaps surprising how little we know about how S. cerevisiae pulls this off.  A new study out in PLOS Genetics by Pais and coworkers sets out to rectify this situation.

The study yielded a number of interesting findings.  It might seem obvious that an organism that can make a lot of ethanol should able to grow in the high-ethanol environment that it created. However, by looking at these characteristics in 68 different S. cerevisiae strains, the authors found that the ability to produce ethanol was at least partially separate at the genetic level from the ability to thrive in it. Pais and coworkers called the first process “high ethanol accumulation capacity” and the second “tolerance of cell proliferation to high ethanol levels.”

Second, they identified DNA differences in three different genes – ADE1, URA3, and KIN3 – that all work together to give certain strains of yeast their high ethanol accumulation capacity.  The most interesting of these three is KIN3.

Kin3p is a protein kinase that has a role in DNA repair.  Since ethanol is a known mutagen, it may be that the DNA differences in the KIN3 gene make its protein better at DNA repair, rescuing the cell from the DNA damage from high ethanol levels.

The authors found these genes as part of a larger study to identify at the genetic level why some strains of S. cerevisiae did better than others in ethanol.  They focused on two strains, CBS1585 and BY710.  CBS1585 is a sake yeast strain that can tolerate and grow in high levels of ethanol while BY710 is a laboratory strain that doesn’t do well with either (although still better than most any other beast out there in nature!).

Yeast holds its liquor way better than this guy.

They created diploids using these two strains, sporulated them into haploids, and then screened these haploids for their ability to deal with high levels of alcohol.  As would be predicted from their survey of the 68 strains, the haploids could be grouped into three distinct but overlapping pools: 

1)   High ethanol accumulation capacity in the absence of cell proliferation

2)   Cell proliferation at high ethanol levels

3)   Poor tolerance and growth in high ethanol

The authors then used pooled-segregant whole-genome sequence analysis to identify the DNA regions critical to the first two functions.  Basically this is just what it sounds like.  They isolated DNA from the three pools and looked for differences in pools 1 and 2 that weren’t in 3.  (OK that is dangerously simplified, but that is the gist of it.)

This is how they identified ADE1URA3, and KIN3 as important in high ethanol accumulation capacity.  We will have to wait for them to pinpoint the important genes in the regions they identified for pool 2 to begin to understand why some strains can proliferate in the presence of high alcohol concentrations.

Once they have identified all of the genes that make certain yeast strains so good at dealing with alcohol, we may be able to engineer a yeast that can make more ethanol for less money.  We can make a great little alcohol producer even better!  

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

Categories: Research Spotlight

Tags: biofuel, brewing, ethanol tolerance, Saccharomyces cerevisiae

Thinking Inside the Box (or Mitochondrion)

March 28, 2013

The mantra in real estate is that location is everything. The same may be true for some cases of bioengineering. You may not get the best yield unless a whole pathway is in the right cellular compartment.

cat in a box

Sometimes it helps to think inside the box.

This is what Avalos and coworkers found for synthesizing branched chain alcohols in the yeast Saccharomyces cerevisiae. These scientists were able to increase yield by 260% by putting the whole pathway of enzymes into the mitochondrion. This is way better than anything anyone else has been able to achieve.

This matters because branched alcohols like isobutanol may prove to be better biofuels than ethanol. We can get more energy out of isobutanol than we can out of ethanol…it has more bang for the buck. Good idea in theory, but producing large quantities of isobutanol has not worked too well in practice.

Yeast is just not very good at making these alcohols, and efforts to improve yields have been anything but inspiring so far. Overexpressing the enzymes in the metabolic pathways that generate isobutanol increased yield by only about 10%. Unfortunately, 10% of almost nothing is still pretty close to nothing.

One of the key metabolic pathways involved in generating isobutanol and other branched chain alcohols is split between the mitochondria and the cytoplasm. Normally, the valine biosynthesis pathway converts pyruvate to valine and alpha-ketoisovalerate in the mitochondria; then those two intermediates, after transport to the cytoplasm, are further converted to isobutanol by the Erlich pathway for valine degradation. Avalos and coworkers reasoned that the failure to increase yield might be because of some rate limiting step in getting the intermediates from the mitochondria to the cytoplasm. And it looks like they may have been right.

They compared the effects of overexpressing the pathway enzymes in the cytoplasm and mitochondria and found the mitochondrial approach won hands down. Overexpression in the cytoplasm bumped yield up 10% while overexpression bumped it up 260%. And this increase wasn’t just for isobutanol. Yields of two other energy rich alcohols, isopentanol and 2-methyl-1-butanol, also went up significantly.

Part of the explanation almost certainly has to do with transport of intermediates between the mitochondrion and the cytoplasm, but that may not be the whole story. The mitochondrion might be a useful environment for other reasons too. For example, its smaller volume means an increase in the concentration of reactants, and its higher pH, lower oxygen content, and more reducing redox potential may be better for certain reactions. It also contains many key intermediates like heme, steroids, biotin and so on.

On the way to improving isobutanol yield, these scientists made it easier for others to test whether moving their pathway to the mitochondria can help increase the yield of their favorite metabolite. Avalos and coworkers created a system of plasmids that easily allows researchers to attach the N-terminal mitochondrial localization signal from Cox4p, subunit IV of the yeast cytochrome c oxidase, to genes of their choice. This will make it much simpler to test whether a pathway’s yield is enhanced by moving it into the mitochondrion.

These results show there is more to increasing yield than overexpression or codon optimization. Sometimes scientists need to take a good hard look at their particular pathway and think outside of the box for new ways to optimize yield. Or sometimes they just need to think within the box that is the mitochondrion.

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

Categories: Research Spotlight

Tags: biofuel, mitochondria, Saccharomyces cerevisiae

Ethanol from Waste

June 16, 2012

Scientists are coming up with ways for yeast to use waste like this to generate ethanol.

Biofuels hold the promise to significantly slow down global warming.  But this will only be the case if they come from something besides corn.

We don’t want them to come from the parts of other plants we eat, either.  Shunting food towards fuel will only jack up food prices and put the lives of the poorest at risk.  Policy makers should not have to decide between feeding the poor and running their cars.

No, to make biofuels worth our time, we need to be able to turn agricultural waste, grass, saplings, etc. into ethanol.  Unfortunately this stuff is mostly cellulose and lignin and we don’t have anything that can efficiently ferment this “lignocellulosic biomass.” 

Many groups are working towards creating strains of Saccharomyces yeast, the predominant fungal organism used for large-scale industrial processes, to do this job.  None have yet been created that can do the job well enough to be industrially viable. They are either poor fermentors or are genetically modified so that they include non-yeast genes.  Ideally any strain would include only Saccharomyces genes, to avoid the public’s fear and loathing of genetically modified organisms.  

This is where a new study in GENETICS by Schwartz and coworkers comes in.  This group is working towards engineering a yeast that can ferment the pentoses like xylose that make up a good chunk of this otherwise inedible biomass, using genes that are naturally occurring in Saccharomyces.  They haven’t yet created such a yeast, but they have at least identified a couple of key genes involved in utilizing xylose.

The researchers took what seemed to be a straightforward approach.  Collect and screen various yeast strains for their ability to grow on xylose and isolate the relevant gene(s) from the best of them.  Sounds easy enough except that most of the strains they’ve found are terrible sporulators.  This means that they couldn’t use conventional methods to isolate the genes they were interested in and so had to come up with new methods.

First they needed to find some way to get the strain to sporulate.  They were able to force sporulation by creating a tetraploid intermediate between the xylose fermenting strain, CBS1502, and the reference strain, CBS7001, by adding an inducible HO gene.  During this process, they noticed that the ability to utilize xylose segregated in a 3:1 pattern.  This usually means that two genes are involved.

They next needed a way to identify these two genes.  What they did was to pool 21 spores that could ferment xylose and 21 that could not.  They then purified the DNA from each pool and compared them using high throughput sequencing.  They eventually found two genes that were key to getting this yeast to use xylose as its carbon source.  (They also found at least two other “bonus” genes that seemed to boost its ability to use xylose).

One of the genes, GRE3, was a known member of a xylose utilization pathway.  But the other gene, the molecular chaperone APJ1, was not known to be involved in metabolizing xylose.  The authors hypothesize that APJ1 might stabilize the GRE3 mRNA.

These two genes may not be enough to create an industrially viable, xylose fermenting Saccharomyces just yet.  But the novel methods of gene isolation presented in this study may allow researchers to find additional genes that might one day get them there.  Then we will have a way to get ethanol without the large carbon footprint and without the human cost.


A genetic engineering approach to getting yeast to ferment agricultural waste

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

Categories: Research Spotlight

Tags: biofuel, biomass, ethanol, fermentation, lignocellulosic biomass, Saccharomyces, Saccharomyces cerevisiae, yeast