AEM Accepts, published online ahead of print on 23 October 2009

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Appl. Environ. Microbiol. doi:10.1128/AEM.01649-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Resistance of Saccharomyces cerevisiae to high furfural concentration is based on NADPH-dependent reduction by at least two oxireductases

Dominik Heer, Daniel Heine, and Uwe Sauer^*

ETH Zurich, Institute of Molecular Systems Biology, Wolfgang-Pauli-Str. 16, 8093 Zurich, Switzerland

^* To whom correspondence should be addressed. Email: sauer{at}imsb.biol.ethz.ch.

Biofuels derived from lignocellulosic biomass hold promises for a sustainable fuel economy, but several problems hamper their economical feasibility. One important problem is the presence of toxic compounds in processed lignocellulosic hydrolysates with furfural as a key toxin. While Saccharomyces cerevisiae has some intrinsic ability to reduce furfural to the less toxic furfuryl alcohol, higher resistance is necessary for process conditions. By comparing an evolved, furfural resistant strain and its parent in micro-aerobic, glucose-limited chemostats at increasing furfural challenge, we elucidate key mechanism and the molecular basis of both natural and high-level furfural resistance. At lower furfural concentrations, NADH-dependent oxireductases are the main defence mechanism. At concentrations above 15 mM, however, [1-¹³C]-flux and global array-based transcript analysis demonstrated that the NADPH-generating flux through pentose-phosphate pathway increases and that NADPH-dependent oxireductases became the major resistance mechanism. The transcript analysis further revealed that iron transmembrane transport is up-regulated in response to furfural. While these responses occur in both strains, high-level resistance in the evolved strain was based on strong induction of ADH7, the uncharacterised ORF YKL071W and 4 further, likely NADPH-dependent oxireductases. By overexpressing the ADH7 gene and the ORF YKL071W, we inverse engineered significantly increased furfural resistance in the parent strain, thereby demonstrating these two enzymes to be key elements of the resistance phenotype.