Resumen de Improved Xylose Metabolism by a CYC8 Mutant of Saccharomyces cerevisiae

Jeroen G. Nijland, Hyun Yong Shin, Leonie G.M. Boender, Paul P. de Waal, Paul Klaassen, Arnold J. M. Driessen

• Engineering Saccharomyces cerevisiae for the utilization of pentose sugars is an important goal for the production of second-generation bioethanol and biochemicals. However, S. cerevisiae lacks specific pentose transporters, and in the presence of glucose, pentoses enter the cell inefficiently via endogenous hexose transporters (HXTs). By means of in vivo engineering, we have developed a quadruple hexokinase deletion mutant of S. cerevisiae that evolved into a strain that efficiently utilizes D-xylose in the presence of high D-glucose concentrations. A genome sequence analysis revealed a mutation (Y353C) in the general corepressor CYC8, or SSN6, which was found to be responsible for the phenotype when introduced individually in the nonevolved strain. A transcriptome analysis revealed altered expression of 95 genes in total, including genes involved in (i) hexose transport, (ii) maltose metabolism, (iii) cell wall function (mannoprotein family), and (iv) unknown functions (seripauperin multigene family). Of the 18 known HXTs, genes for 9 were upregulated, especially the low or nonexpressed HXT10, HXT13, HXT15, and HXT16. Mutant cells showed increased uptake rates of D-xylose in the presence of D-glucose, as well as elevated maximum rates of metabolism (Vmax) for both D-glucose and D-xylose transport. The data suggest that the increased expression of multiple hexose transporters renders D-xylose metabolism less sensitive to D-glucose inhibition due to an elevated transport rate of D-xylose into the cell.