Adaptive evolution of yeast

BioTork and NCERC make progress in developing commercially applicable xylose-fermenting yeast
By Ron Kotrba | May 10, 2012

Gainesville, Fla.-based BioTork and the National Corn to Ethanol Research Center at Southern Illinois University in Edwardsville announced completion of the first step in a joint development program intended to improve the processes and economics of ethanol production.

In the first step of the program, BioTork used proprietary technology developed by Evolugate to facilitate, via adaptive evolution, growth optimization of yeast genetically engineered by USDA. The result is a strain of yeast that can grow on xylose as the sole carbon source with a growth rate three times faster.

“While improvements to the growth rate and initial scale-up of its performance in an industrial setting are underway, this strain has the potential to be one of the first economically viable xylose-fermenting strains, and represents a fruitful combination of genetic engineering and adaptive evolution,” says Tom Lyons, BioTork chief scientific officer.

“This project is crucial for corn farmers, ethanol producers and for gas prices at the pump,” says Sabrina Trupia, director of biological research at NCERC. “During the last decade, the ethanol industry has been focusing on the use of lignocellulosic biomass as low-cost and abundant feedstock. Different agricultural residues have been considered such as corn fiber, corn stover, straw and bagasse but the stumbling block to commercial success has been the inability of most yeast strains to ferment the complex sugars in the lignocellulosic biomass.”

Xylose is the second most abundant sugar in biomass after glucose, making up nearly 30 percent of its dry weight. The yeast Saccharomyces cerevisiae has been a workhorse for high-yield ethanol production for centuries, but it can’t ferment xylose.

The USDA genetically engineered a strain of S. cerevisiae to be able to ferment xylose, however, as is often the case with genetic engineering, the modification left it growth-attenuated, with a generation time of more than 20 hours on xylose. Thus, even though the strain can convert xylose to ethanol, it cannot do so with a high enough time-space yield to be of economic value.

“The reason for this is simple,” says Lyons, “in many instances genetic engineering produces strains that are 'competent' to do the job for which they were engineered at a laboratory scale, but incapable of doing so at a commercially viable scale.”

The achievement presents several opportunities for the biofuel industry. First, it can generate immediate improvements for ethanol producers: with the glucose and xylose available in distillers dried grains, ethanol production in the U.S. can be increased by approximately 10 percent, or 1.3 billion gallons with an estimated value of $3.2 billion. With the right microorganisms, all the sugars in DDGs as well as other lingocellulosic corn residues can be fermented yielding up to 16 billion gallons of additional ethanol with the existing corn harvest. Second, since as much as 30 percent of all cellulosic biomass is comprised of the pentose sugar D-xylose, this proof of principle will open the door to the use a variety of biomass for ethanol production. And third, it illustrates how genetic engineering and adaptive evolution complement each other, lending credence to the principle that genetically engineered microbes could be optimized through adaptive evolution for performance in the real world.