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Genome Sequenced of Most Efficient Xylose-Fermenting Yeast; Implications for Cellulosic Ethanol Production

Pichia stipitis. Source: JGI.

Researchers from the US Department of Energy Joint Genome Institute (DOE JGI) and collaborators at the US Forest Service, Forest Products Laboratory (FPL) have sequenced the genome of the yeast Pichia stipitis. This yeast has the highest native capacity of any known microbe for fermentation of the five-carbon sugar xylose that is abundant in cellulosic biomass.

The research, entailing the identification of numerous genes in P. stipitis responsible for its fermenting and cellulose-bioconverting prowess, and an analysis of these metabolic pathways, is featured in the 4 March advanced online publication of Nature Biotechnology.

Increasing the capacity of P. stipitis to ferment xylose and using this knowledge for improving xylose metabolism in other microbes, such as Saccharomyces cerevisiae, brewer’s yeast, offers a strategy for improved production of cellulosic ethanol. In addition, this strategy could enhance the productivity and sustainability of agriculture and forestry by providing new outlets for agricultural and wood harvest residues.

—Eddy Rubin, DOE JGI Director

Ligonocellulosic biomass—a complex of cellulose, hemicellulose, and lignin—is derived from such plant-based feedstocks as agricultural waste, paper and pulp, wood chips, grasses, or trees such as poplar, recently sequenced by DOE JGI.

Under current strategies for generating lignocellulosic ethanol, these forms of biomass require expensive and energy-intensive pretreatment with chemicals and/or heat to loosen up this complex. Enzymes are then employed to break down complex carbohydrate into sugars, such as glucose and xylose, which can then be fermented to produce ethanol. Additional energy is required for the distillation process to achieve a fuel-grade product.

The information embedded in the genome sequence of Pichia has helped us identify several gene targets to improve xylose metabolism. We are now engineering these genes to increase ethanol production.

—Thomas W. Jeffries of the Forest Products Laboratory

According to the JGI, P. stipitis is unusual in that it is the best of only a few yeasts known to ferment xylose to ethanol in high yield. It is related to several yeasts that are found as endosymbionts of beetles that inhabit and degrade white-rotted wood. Unlike Saccharomyces cerevisiae, which regulates fermentation by sensing the presence of fermentable sugars such as glucose, Pichia stipitis induces fermentative activity in response to oxygen limitation.

Jeffries said that yeast strains like Pichia have evolved to cope with the oxygen-limited environment rich in partially digested wood that is encountered in the gut of insects, from where the sequenced strain was originally isolated.

FPL has a Cooperative Research and Development Agreement (CRADA) in place with a New York City-based bioenergy company, Xethanol Corporation, which plans to integrate Dr. Jeffries’ findings into its large-scale biofuels production processes.

Pichia joins white rot fungus in the growing portfolio of bioenergy-relevant genomes sequenced by DOE JGI through its user programs and contributed freely to the worldwide scientific community.



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