Isobutanol is a superior biofuel compared to ethanol, and it is naturally produced by yeasts. Previously, Saccharomyces cerevisiae has been genetically engineered to improve isobutanol production. We found that yeast cells engineered for a cytosolic isobutanol biosynthesis secrete large amounts of the intermediate 2,3-dihydroxyisovalerate (DIV). This indicates that the enzyme dihydroxyacid dehydratase (Ilv3) is limiting the isobutanol pathway and/or yeast exhibit effective transport systems for the secretion of the intermediate, competing with isobutanol synthesis. Moreover, we found that DIV cannot be taken up by the cells again. To identify the responsible transporters, microarray analysis was performed with a DIV producing strain compared to a wild type. Altogether, 19 genes encoding putative transporters were upregulated under DIV-producing conditions. Thirteen of these were deleted together with five homologous genes. A yro2 mrh1 deletion strain showed reduced DIV secretion, while a hxt5 deletion mutant showed increased isobutanol production. However, a strain deleted for all the 18 genes secreted even slightly increased amounts of the intermediates and less isobutanol. The lactate transporter Jen1 turned out to transport the intermediate 2-ketoisovalerate, but not DIV. The results suggest that the transport of DIV is a rather complex process and several unspecific transporters seem to be involved.
Keywords: Ehrlich pathway; Ethanol Red; branched-chain amino acid pathway; isobutanol; metabolite secretion; xylose fermentation.
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