Appetite for Improvement

Researchers examine why certain yeasts like to eat C5 sugars and others don’t
By Kris Bevill | August 15, 2011

Using Mother Nature as their teacher, researchers at the U.S. DOE’s Great Lakes Bioenergy Research Center, along with the DOE’s Joint Genome Institute, have sequenced the genomes of two types of yeasts found in bark beetles and then compared and contrasted the results with other yeasts’ genome sequences to determine which can best convert the elusive C5 sugars of cellulosic material to ethanol. What they found surprised and encouraged them, and left them with more than enough work to fill the next several years.

Yeasts currently available for biofuel production eat C5 sugars, specifically xylose, slowly and inefficiently, if at all. Because much of the sugar in hemicellulose is comprised of those five-carbon sugars, ethanol yields are limited without a yeast that has an appetite for C5s, according to Audrey Gasch, associate professor of genetics at the University of Wisconsin-Madison and principal investigator. “That’s one of the big bottlenecks in making cellulosic ethanol economically feasible,” she says. Researchers have explored this issue for decades, but most studies have focused on individual strains. This time, researchers decided to examine some of nature’s best xylose eaters—yeasts found in the hindgut of wood-eating bark beetles—and compare them with other yeasts to try to understand why some like C5 sugars and some don’t.

“By comparing the genome sequences and expression patterns of many yeasts—rather than just looking at one—we were able to identify elements common to all xylose-fermenting yeasts, and elements absent from nonxylose fermenting yeasts,” says Dana Wohlbach, a postdoctoral researcher at the UW-Madison and lead author. “We were expecting to see pretty distinct differences, but actually what we found is that in terms of gene expression, they were very similar.” This finding indicates that the ability of certain yeasts to consume xylose has been lost relatively recently and that the genes to recognize the sugar may still be present. “From an ecological and evolutionary standpoint, that was very interesting,” Wohlbach says.

The researchers identifed 40 genes of interest and tested 10 genes so far to find two that improved a nonxylose-consuming yeast’s ability to eat that sugar. “Twenty percent is a much higher success rate than any of us thought,” Gasch says. Many years of testing lie ahead, but researchers believe there could be a hidden gold mine somewhere in the data just waiting to be discovered. While most of the work so far has been conducted at the petri dish scale using pure, liquid sugars, a few tests have been run on corn stover that has been pre-treated using the University of Michigan’s AFEX process. Researchers would like to conduct more tests in industrial conditions using various feedstocks, because preprocessing treatments used for cellulosic materials can be very stressful for the yeasts and will impact their ability to consume the sugars. Wohlbach says the ideal yeast will be one that consumes both C5 and C6 sugars simultaneously. “That would be the Holy Grail,” she says.

Gasch points out that just as there will be a need for a host of pretreatment processes to accommodate various feedstocks, a variety of yeasts will be needed to suit those systems. Gasch believes that they might be able to make their first C5-eating yeast strain available to industry within the next year. “We would like to do more work on it, but our goal is to make an impact for the country so certainly we’ll make the strain available,” she says. 

—Kris Bevill