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Yeast is rising as a biofuel booster


A whole yeast cell (Saccharomyces cerevisiae) is viewed by X-ray microscopy. Inside, the nucleus and a large vacuole (shown in red) are visible.

Researchers say they've used genetic engineering to create a strain of yeast that can cut the time needed to make ethanol from cellulosic sources in half. It's the latest twist in efforts to fine-tune microbes for "frankenfuel" production.

Like Frankenstein's monster, these ethanol-producing organisms draw upon genetic combinations not found in nature. The scientists reporting their results today in the Proceedings of the National Academy of Sciences started out with common brewer's yeast, then adapted a few tricks used by a different strain of yeast as well as a cellulose-loving fungus.

The original yeast, Saccharomyces cerevisiae, is quite good at fermenting glucose, which is currently the primary sugar converted to ethanol in the industrial fermentation process. This is the process by which yeast makes bread rise, and by which yeast turns fruit and grain into wine, beer and other alcoholic beverages. But energy companies would rather make ethanol from cellulosic materials (such as wood waste and switchgrass) rather than from edible products (such as corn and sugar cane). So anything that raises the efficiency of cellulosic ethanol production makes biofuels look more attractive as a long-term energy solution.

One of the big problems is that glucose is only one of the sugars contained in cellulosic material. Brewer's yeast can't ferment the other major type of sugar, known as xylose. "Xylose is a wood sugar, a five-carbon sugar that is very abundant in lignocellulosic biomass but not in our food," Yong-Su Jin, a professor of food science and human nutrition at the University of Illinois, said in a news release. "Most yeast cannot ferment xylose."

Even if a yeast strain can handle xylose fermentation, it won't start in on the xylose until all the glucose is gone. "It's like giving meat and broccoli to my kids," Jin explained. "They usually eat the meat first and the broccoli later."

Jin and his colleagues — including researchers from the University of Illinois, Lawrence Berkeley National Laboratory, the University of California at Berkeley, Seoul National University and the energy company BP — inserted genes from a xylose-converting yeast to give S. cerevisiae the power to turn xylose into ethanol. They also added the capability of a fungus known as Neurospora crassa to work with a precursor of glucose known as cellobiose.

The combination of those two tricks, plus some extra tweaks, enabled the franken-yeast to ferment cellobiose and xylose at the same time. That avoided the glucose vs. xylose, meat vs. broccoli problem.

"If you do the fermentation by using only cellobiose or xylose, it takes 48 hours," Suk-Jin Ha, a postdoctoral researcher at the University of Illinois and the study's lead author, said in today's release. "But if you do the co-fermentation with the cellobiose and xylose, double the amount of sugar is consumed in the same amount of time and [the process] produces more than double the amount of ethanol."

The new yeast strain is at least 20 percent more efficient at converting xylose to ethanol than other strains, Jin said.

He said the potential cost benefits are significant: "We don't have to do two separate fermentations. We can do it all in one pot. And the yield is even higher than the industry standard. We are pretty sure that this research can be commercialized very soon."

This approach builds upon research published in September on the journal Science's website — and one of the researchers involved in that earlier study, Berkeley's Jamie Cate, played a role in the newly published study as well. As I noted back in September, other researchers are working on different ways to use yeast for producing biofuel. Bottom line? If cellulosic ethanol ever becomes a major part of America's energy equation, it's sounding as if genetically modified yeast will be the key that turns the ignition.

But what do you think? How will biofuel fit in alongside fossil fuels, solar and wind energy, nuclear power and other options? Feel free to discuss America's energy future in the comment space below.

In addition to Jin, Ha and Cate, the authors of the PNAS paper, titled "Engineered Saccharomyces Cerevisiae Capable of Simultaneous Cellobiose and Xylose Fermentation," include Jonathan Galazka, Soo Rin Kim, Jin-Ho Choi, Xiaomin Yang, Jin-Ho Seo and N. Louise Glass. The research was supported by the Energy Biosciences Institute, a BP-funded initiative.

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