PHO13 deletion-induced transcriptional activation prevents sedoheptulose accumulation during xylose metabolism in engineered Saccharomyces cerevisiae

The deletion of PHO13 (pho13 Delta) in Saccharomyces cerevisiae, encoding a phosphatase enzyme of unknown specificity, results in the transcriptional activation of genes related to the pentose phosphate pathway (PPP) such as TAL1 encoding transaldolase. It has been also reported that the pho13 Delta mutant of S. cerevisiae expressing a heterologous xylose pathway can metabolize xylose efficiently compared to its parental strain. However, the interaction between the pho13 Delta-induced transcriptional changes and the phenotypes of xylose fermentation was not understood. Thus we investigated the global metabolic changes in response to pho13 Delta when cells were exponentially growing on xylose. Among the 134 intracellular metabolites that we identified, the 98% reduction of sedoheptulose was found to be the most significant change in the pho13 Delta mutant as compared to its parental strain. Because sedoheptulose-7-phosphate (S7P), a substrate of transaldolase, reduced significantly in the pho13 Delta mutant as well, we hypothesized that limited transaldolase activity in the parental strain might cause dephosphorylation of S7P, leading to carbon loss and inefficient xylose metabolism. Mutants overexpressing TAL1 at different degrees were constructed, and their TAL1 expression levels and xylose consumption rates were positively correlated. Moreover, as TAL1 expression levels increased, intracellular sedoheptulose concentration dropped significantly. Therefore, we concluded that TAL1 upregulation, preventing the accumulation of sedoheptulose, is the most critical mechanism for the improved xylose metabolism by the pho13 Delta mutant of engineered S. cerevisiae. (C) 2015 International Metabolic Engineering Society. Published by Elsevier Inc. All rights reserved.