MADISON – Scientists at the University of Wisconsin–Madison and the Great Lakes Bioenergy Research Center (GLBRC) have found a way to nearly double the efficiency with which Saccharomyces cerevisiae, a commonly used industrial yeast strain, converts plant sugars to biofuel. The newly engineered “super yeast” could boost the economics of making ethanol, specialty biofuels and bioproducts.

Though S. cerevisiae has been the baker and brewer’s yeast of choice for centuries, it poses a unique challenge to researchers using it to make biofuel from cellulosic biomass such as grasses, woods, or the non-food portion of plants. The world-famous microbe is highly adept at converting a plant’s glucose to biofuel but is otherwise a picky eater, ignoring the plant’s xylose, a five-carbon sugar that can make up nearly half of all available plant sugars.

“For cellulosic biofuels to become economically feasible, microbes need to be able to convert all of a plant’s sugars, including xylose, into fuel,” says Trey Sato, the GLBRC study’s lead researcher and a UW–Madison associate scientist.

In a study published today (Oct. 14, 2016) in the journal PLOS Genetics, Sato 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. To uncover these genetic mutations, the researchers had to untangle millions of years of evolution, teasing out what led S. cerevisiae to become so selective in its eating habits in the first place.

First, Sato and colleagues 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 ten 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.