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Crop Biotech Update

Improvement of L-Arabinose Utilization in Pentose-fermenting Saccharomyces cerevisiae

October 14, 2011

In the production of ethanol from lignocellulosic biomass, the major simple sugars after biomass pretreatment (lignin removal) and saccharification (breakdown of complex carbohydrates in pretreated biomass) are usually (1) glucose (a 6-carbon sugar or hexose), (2) xylose,and (3) arabinose (the latter two are 5-carbon sugars, or pentoses). These sugars are usually contained in a liquid stream called, "hydrolyzates", which are fermented to ethanol by the yeast, Saccharomyces cerevisiae. In this system, only the glucose is effectively utilized, since Saccharomyces cerevisiae cannot utilize pentoses. Hence, not all carbohydrates in the biomass are converted to ethanol. To address this problem, pentose-utilizing strains of this yeast have been developed, but many are limited by the slow transport of these pentoses into the cells during fermentation. One strategy is to find ways to increase pentose transport in pentose-fermenting Saccharomyces cerevisiae.

Researchers from Institute of Molecular Biosciences, Goethe-University Frankfurt am Main (Germany) report the "cloning and characterization of two sugar transporters, AraT from the yeast Scheffersomyces stipitis and Stp2 from the plant Arabidopsis thaliana, which mediate the uptake of L-arabinose but not of D-glucose into S. cerevisiae cells". From their new screening system, they were able to identify two heterologous sugar transporters which can support uptake and utilization of L-arabinose in L-arabinose fermenting S. cerevisiae cells, especially at low L-arabinose concentrations. The full study is published in the open-access journal, Biotechnology for Biofuels (URL above).