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.

activity

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