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Background  Hydrolysates of plant biomass used for the production of lignocellulosic biofuels typically contain sugar mixtures consisting mainly of D-glucose and D-xylose, and minor amounts of L-arabinose. The yeast  Saccharomyces cerevisiae  is the preferred microorganism for the fermentative production of ethanol but is not able to ferment pentose sugars. Although D-xylose and L-arabinose fermenting  S. cerevisiae  strains have been constructed recently, pentose uptake is still a limiting step in mixed sugar fermentations.    Results  Here we described 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. A yeast strain lacking all of its endogenous hexose transporter genes and expressing a bacterial L-arabinose utilization pathway could no longer take up and grow with L-arabinose as the only carbon source. Expression of the heterologous transporters supported uptake and utilization of L-arabinose especially at low L-arabinose concentrations but did not, or only very weakly, support D-glucose uptake and utilization. In contrast, the  S. cerevisiae  D-galactose transporter, Gal2, mediated uptake of both L-arabinose and D-glucose, especially at high concentrations.    Conclusions  Using a newly developed screening system we have identified two heterologous sugar transporters from a yeast and a plant which can support uptake and utilization of L-arabinose in L-arabinose fermenting  S. cerevisiae  cells, especially at low L-arabinose concentrations.

biotechnology for biofuels

Improving L-arabinose utilization of pentose fermenting Saccharomyces cerevisiae cells by heterologous expression of L-arabinose transporting sugar transporters