Fungus Genes Help Turn Grass into Ethanol

Modified yeast could help make ethanol from hard-to-digest materials.

Genes copied from a common fungus could simplify the production of ethanol from abundant materials such as grass and wood chips, a development that could one day help ethanol compete with gasoline.

The transporters: Fungus proteins that help transport complex sugars for digestion can be seen in this image of yeast. The transporter proteins have been tagged with a green fluorescent protein.

Scientists have taken genes from a fungus that grows on grass and dead plants, and transplanted them into yeast that is already used to turn sugar into ethanol. The genes let the yeast ferment parts of plants that it normally can’t digest, potentially streamlining the production of ethanol.

“It’s just a more efficient process,” says Jamie Cate, a biologist at the University of California, Berkeley and at Lawrence Berkeley National Laboratory. “Shaving off every dime that you can could make it compete with oil,” says Cate, who led the work.

Most ethanol is produced using simple sugars, like the glucose derived from corn kernels or sugar cane. Ethanol producers would like to use glucose from more abundant sources, such as corn husks and stalks, switchgrass, wood waste, and other tough plant materials. But those plant parts are made of cellulose, a carbohydrate built from long chains of sugars. For yeast to produce ethanol from these materials, the complex carbohydrate has to first be broken down into very simple sugars, a process that takes time and normally requires the addition of expensive enzymes.

With the new technique, ethanol makers would no longer have to break cellulose down into simple sugars. Instead, they would only need to break down cellulose into an intermediate material called cellodextrin. The modified yeast can work with this, instead of waiting for it to be broken down all the way to glucose, removing steps that cost time and money.

Yeast takes a simple molecule such as glucose and digests it as food, producing alcohol as a by-product. The Berkeley researchers, along with a colleague from the Chinese Academy of Sciences in Tianjin, found that a fuzzy orange fungus called Neospora crassa that grows on dead plant matter produces two different proteins that help transport more complex cellulose molecules into cells for digestion. In addition, they found that the fungus produces an enzyme that can help further break down those molecules. The researchers then pored through the genome of a Neospora crassa to find the genes responsible for these abilities

Lee Lynd, an environmental engineer at Dartmouth, says the concept of engineering sugar-fermenting microbes so they’ll also produce enzymes “is widely regarded as the most promising approach” for converting cellulosic materials into ethanol. Many researchers are working on consolidating ethanol processing steps, he says, and some have achieved better results in some parts of the process. But Lynd says this is the first time, as far as he knows, that someone has cloned these transporters.

“These advances are relevant, demonstrate in principle the promise of engineering microbes for improved biomass processing, and could be applied commercially,” Lynd says. “However, the advances are not enabling by themselves, and represent a relatively early step on a long path.”

The technique doesn’t address much of the processing involved in ethanol production. Ethanol makers would still need to use enzymes to break the cellulose down to an intermediate stage called cellodextrin. But the yeast can work with this, instead of waiting for it to be broken down all the way to glucose, removing steps that cost time and money.

For their research, the group used a strain of yeast commonly studied in laboratories. The genes will have to be inserted into strains bred to withstand the demands of industrial ethanol production. Scientists at the University of Illinois, part of the Energy Biosciences Institute that funded the research, will work on that. Meanwhile, Cate will continue to study Neospora to see if he can find an even better combination of genes. “We’re still going to be poking and prodding at Neospora to see what other tricks it might have for us,” he says.

It could be five years before the modified yeast is ready for use in a demonstration-scale ethanol plant, and perhaps a decade before ethanol made this way winds up in gas tanks, Cate says. Researchers won’t know for some time how much of a boost in yield the modified yeast will produce until it is tried in a production setting.

“We make a 10 to 20 percent improvement, other companies make a 10 to 20 percent improvement in their enzymes, and all of a sudden we’ve brought down the cost to where it can start to be competitive with oil,” says Cate.

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