Functional characterization of a highly specific l-arabinose transporter from Trichoderma reesei

Abstract

Abstract Background Lignocellulose biomass has been investigated as a feedstock for second generation biofuels and other value-added products. Some of the processes for biofuel production utilize cellulases and hemicellulases to convert the lignocellulosic biomass into a range of soluble sugars before fermentation with microorganisms such as yeast Saccharomyces cerevisiae . One of these sugars is l -arabinose, which cannot be utilized naturally by yeast. The first step in l -arabinose catabolism is its transport into the cells, and yeast lacks a specific transporter, which could perform this task. Results We identified Trire2_104072 of Trichoderma reesei as a potential l -arabinose transporter based on its expression profile. This transporter was described already in 2007 as d -xylose transporter XLT1. Electrophysiology experiments with Xenopus laevis oocytes and heterologous expression in yeast revealed that Trire2_104072 is a high-affinity l -arabinose symporter with a K m value in the range of $$\sim$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>∼</mml:mo></mml:math> 0.1–0.2 mM. It can also transport d -xylose but with low affinity ( K m $$\sim$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>∼</mml:mo></mml:math> 9 mM). In yeast, l -arabinose transport was inhibited slightly by d -xylose but not by d -glucose in an assay with fivefold excess of the inhibiting sugar. Comparison with known l -arabinose transporters revealed that the expression of Trire2_104072 enabled yeast to uptake l -arabinose at the highest rate in conditions with low extracellular l -arabinose concentration. Despite the high specificity of Trire2_104072 for l -arabinose, the growth of its T. reesei deletion mutant was only affected at low l -arabinose concentrations. Conclusions Due to its high affinity for l -arabinose and low inhibition by d -glucose or d -xylose, Trire2_104072 could serve as a good candidate for improving the existing pentose-utilizing yeast strains. The discovery of a highly specific l -arabinose transporter also adds to our knowledge of the primary metabolism of T. reesei . The phenotype of the deletion strain suggests the involvement of other transporters in l -arabinose transport in this species.

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