For hundreds of years, the yeast species Saccharomyces cerevisiae has helped humans bake bread and brew beer. More recently, it has also assisted in the fight against climate change. Researchers from the University of Wisconsin-Madison have created a variety of the microbe that produces the biofuel ethanol in a more eco-friendly and efficient way than past methods. The study was published last month in PLOS Genetics.
However, while baker’s yeast is a glutton for the sugar molecule glucose, it cannot normally digest xylose, another sugar that composes up to half of some plants. According to a 2012 study from the U.S. Department of Energy’s Argonne National Laboratory, producing and burning ethanol made from glucose-rich corn kernels reduces greenhouse gas emissions by 19–48% compared to traditional gasoline.
But ethanol made from more xylose-heavy crops like switchgrass, which require less energy and other resources to grow, could cut emissions by 90–115%, with the latter figure taking into account other parts of the plants that could be burned instead of fossil fuels to produce electricity.
While scientists have used genetic engineering to turn yeast into xylose eaters, the microbes still didn’t do it nearly as well as other microorganisms. Unfortunately, these less-picky microbes “are often less reliable workers” in industrial settings, said Douglas Tiffany, an assistant extension professor of agribusiness at the University of Minnesota who was not associated with the study. If baker’s yeast were better at processing xylose, turning xylose-heavy crops into ethanol would become more economical. Because facilities that do this are far more expensive to build than their glucose-focused counterparts, creating such yeast is “fundamental and very necessary” to making those factories viable, Tiffany said.
The researchers created xylose-digesting yeast by injecting them with the genes that grant other organisms that capability and adding a procedure called directed evolution. This step relies on the fact that microorganisms like yeast reproduce extremely quickly, so natural selection can change them drastically in a short amount of time.
The team took yeast that had been given xylose-processing genes and then fed them nothing else, making them dependent on xylose for survival. Over time, the few yeast that were able to get enough energy from the sugar multiplied and their children acquired their xylose-digesting genes as well as random genetic changes that are a natural part of reproduction. While many mutations are neutral or harmful, if you wait long enough, a beneficial change is likely to emerge, spurring a sudden increase in growth.
“What that usually means is that one cell got the right combination of mutations that gave it a fitness advantage and allowed it to propagate much more rapidly than its siblings,” said Trey Sato, the study’s first author and an associate scientist at the University of Wisconsin-Madison’s Wisconsin Energy Institute. In this case, that advantage was an improved ability to consume xylose.
Sequencing the genome of this new, xylose-eating yeast and comparing it to the first generation revealed four mutations that play a role in xylose processing, three of which were not previously known to be involved. Putting those four altered genes into the old yeast gave them the same ability, although it remains unclear precisely how. In addition, one of the mutations caused the yeast to be worse at responding to stress, a limitation that could hinder its ability to produce ethanol.
The research could allow other scientists to transfer those mutated genes into the species they study to quickly and easily turn them into xylose eaters. “There’s no guarantee of course because genes act differently in different species,” Sato said.
Tiffany also thinks the research could be applied to creating yeast that can convert other chemicals found in plants into useful industrial materials. “I think there’s value in this research beyond thinking about fuels,” he said.