Scientific Method / Science & Exploration

Engineering yeast can make biofuels commercially viable

Putting some genes from a cellulose eater into the yeast that makes the …

We've been performing the basic process of producing bioethanol for millennia, but have generally limited our efforts to the production of beer and wine. With the advent of biofuels, it's still possible to use the same general approach—put a yeast and some sugar together without oxygen—to convert some crops to ethanol. The problem comes when switching from sugar to the cellulose in the body of the plant, a source that's easier to obtain in bulk. Our helpful yeast doesn't do cellulose. At least not without a bit of genetic engineering.

Cellulose is a very long polymer of sugar molecules. Yeast will happily use those sugars to produce ethanol, but it can't get at them while they're trapped in cellulose. As a result, a suite of enzymes from other species are first used to break the cellulose down into individual sugars, a step that adds to the cost and complexity of producing the ethanol. A Berkeley-Tianjin collaboration has now cut down on the steps required to get yeast feeding on cellulose.

The initial step in digesting a cellulose molecule involves enzymes that break the long chains into short pieces; these short pieces are later broken down into individual sugars that the yeast can digest. A different fungus, Neurospora crassa, grows happily on cellulose, in part because it can import the short pieces of cellulose and break them down inside the cell. The authors identified the genes responsible for both the import and chemical reactions, and transferred them into yeast cells.

With the new proteins, yeast could be placed directly into the reaction in which enzymes cut up the cellulose into smaller pieces, and readily pump out ethanol. In fact, their ethanol production ended up being over 85 percent of the theoretical maximum rate—and that's before any optimization or evolution of the system had been done. The authors suggest that the newly engineered yeast strain removes a major bottleneck in the production of biofuels from plant matter, and will probably help make these biofuels commercially viable.

Science, 2010. DOI: 10.1126/science.1192838  (About DOIs).

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