Nature leads research team to better yeast

By Kris Bevill | December 27, 2010
Posted Dec. 21, 2010 Researchers at the Great Lakes Bioenergy Research Center have taken a cue from Mother Nature and are evaluating natural strains of yeast to guide the development of industrial strains that will be better able to handle the stressful environments associated with cellulosic and traditional ethanol production. The research group has so far identified certain genes in wild yeast cells that improve their tolerance of ethanol. This information can be used to bioengineer yeast cells that produce higher yields and concentrations of ethanol. The next step is to identify which genes allow some yeast cells to tolerate higher levels of other toxins that are typically found in cellulose. Researchers say that identifying those genes could lead to a significant breakthrough for cellulosic ethanol production. "We were very pleased with how successful we were in identifying genes that are new engineering targets, but it's almost like the tip of the iceberg," said Audrey Gasch, assistant professor of genetics at the University of Wisconsin-Madison. Gasch is also a researcher at the GLBRC and co-author of a study on the group's research that was published in the December issue of Genetics - the Genetics Society of America's scientific journal. She said the inability of yeast cells to cope with toxins found in cellulosic material has been a "major bottleneck" in the task of producing cellulosic ethanol efficiently and is now the focus of the team's research. "The combination of ethanol and these other toxins is kind of a double whammy for the yeast cells," she said. By further examining various strains of natural yeasts and their ability to resist similar toxins, the group could discover what genes can be "switched on" in industrial strains to allow them to better handle cellulosic materials. Examining yeast strains with the goal of engineering more toxin-resistant strains is not a new area of research, Gasch said. However, the GLBRC group employed a new approach to this research, which they believe greatly speeded up the results process. What was their unique approach? They simply studied nature. To do this, the group first obtained a sampling of approximately 50 different strains of the same species of yeast - Saccharomyces cerevisiae - some industrial strains and some wild strains. "Saccharomyces cerevisiae grows in diverse environments from all over the world," Gasch said. "But different strains from different environments have very different tendencies. Some of those strains are just inherently very resistant to toxins that we care about." Samples of natural yeast strains were obtained from a wide range of environments, including the Philippines, Siberia and the research team's local oak tree soil in Wisconsin. Once the sampling of strains had been gathered, the research team compared and contrasted the resistant and less-resistant strains and examined how those strains responded to ethanol. Yeast cells have about 6,000 genes, Gasch said, so by employing this unique compare/contrast method, researchers were able to greatly accelerate the evaluation process. "One of the problems with bioengineering in general is trying to figure out which aspects of cellular physiology to tweak or engineer," she said. "It was the comparison of the tolerant strains and the sensitive strains that allowed us to very quickly hone in on the genes that were allowing the resistant strains to maintain their resistance to ethanol." After identifying which genes were responsible for ethanol resistance, researchers were able to essentially "turn them on" in industrial strains, allowing them to be more tolerant of ethanol and thus boosting their production performance. Gasch hopes subsequent studies will identify many other genes that can also be applied to this engineering. The GLBRC is focused on improving the efficiency of cellulosic ethanol production, and the team is currently working with about 1,000 strains of yeast using the same method as was previously applied to the strains in an attempt to identify genes that can be tweaked to improve tolerance of cellulosic toxins. Gasch said the team anticipates publishing its next round of studies, focused on cellulosic developments, sometime next year. The Great Lakes Bioenergy Research Center is led by the University of Wisconsin-Madison. Michigan State University is a major partner in the center. Additional partners include the U.S. DOE's National Laboratories, Cornell University, Illinois State University, Iowa State University, the University of Minnesota, the University of Toledo, and Lucigen Corp.