UW-Madison and GLBRC team engineers S. cerevisiae to ferment xylose, nearly doubling efficiency of converting biomass sugars to biofuel
Scientists at the University of Wisconsin-Madison and the Great Lakes Bioenergy Research Center (GLBRC) have used directed evolution to nearly double the efficiency with which the commonly used industrial yeast Saccharomyces cerevisiae converts plant sugars to biofuel. The resulting improved yeast could boost the economics of making ethanol, specialty biofuels and bioproducts.
S. cerevisiae poses a challenge to researchers using it to make biofuel from cellulosic biomass such as grasses, woods, or the nonfood portion of plants. Although the microbe is highly adept at converting a plant’s glucose to biofuel, it ignores the plant’s xylose, a five-carbon sugar that can make up nearly half of all available biomass sugars.
The inability of native Saccharomyces cerevisiae to convert xylose from plant biomass into biofuels remains a major challenge for the production of renewable bioenergy. Despite extensive knowledge of the regulatory networks controlling carbon metabolism in yeast, little is known about how to reprogram S. cerevisiae to ferment xylose at rates comparable to glucose.—Sato et al.
In an open-access paper published in the journal PLOS Genetics, Trey Sato, the GLBRC study’s lead researcher and a UW–Madison associate scientist and his GLBRC collaborators describe the isolation of specific genetic mutations that allow S. cerevisiae to convert xylose into ethanol, a finding that could transform xylose from a waste product into a source of fuel.
Sato and colleagues first gave the yeast a choice akin to eating carrots for dinner or nothing at all, surrounding S. cerevisiae with xylose until it either reevaluated its distaste for xylose or died. It took 10 months and hundreds of generations of “directed evolution” for Sato and his colleagues, including co-corresponding authors Robert Landick, a UW–Madison professor of biochemistry, and Audrey Gasch, a UW– Madison professor of genetics, to create a strain of S. cerevisiae that could ferment xylose.
Once the researchers had isolated the super yeast they named GLBRCY128, they also needed to understand exactly how the evolution had occurred in order to replicate it. Gasch compared Y128’s genome to the original strain, combing through the approximately 5,200 genes of each to find four gene mutations responsible for the adapted behavior. To verify their finding, the researchers manually deleted these mutations from the parent strain, producing the same result.
Sato says this work could enable a wide variety of biofuels research going forward. With the technique for making Y128 published, researchers are free to make it themselves for the purposes of applying it to new biomass pretreatment technologies or to different plant materials.
Future research may also focus on the super yeast’s potentially powerful role in creating specialty biofuels and bioproducts.
We want to take this strain and make higher-order molecules that can be further converted into jet fuels or something like isobutanol, lipids or diesel fuel. And if we know how to better metabolize carbon, including xylose, anybody in theory should be able to rewire or change metabolic pathways to produce a variety of biofuel products.—Trey Sato
Sato TK, Tremaine M, Parreiras LS, Hebert AS, Myers KS, Higbee AJ, et al. (2016) “Directed Evolution Reveals Unexpected Epistatic Interactions That Alter Metabolic Regulation and Enable Anaerobic Xylose Use by Saccharomyces cerevisiae.” PLoS Genet 12(10): e1006372 doi: 10.1371/journal.pgen.1006372