Sunday, May 3, 2009
2-29
The Endogenous Molecular Basis For Improved Xylose Utilization of Saccharomyces cerevisiae
Zhimin Li, Nan Liu, and Shulin Chen. Department of Biological System Engineering, Washington State University, L. J. Smith 213, Pullman, WA 99164
Although varied recombinant S.cerevisiae strains capable of utilizing xylose have been constructed, the cofactor and global metabolic imbalance caused by the heterogonous XR/XD genes have been extensively demonstrated. In this study, a mutant strain S.cerevisiae 9763v, without any heterogonous genes, displayed significantly improved ability to utilize xylose. The growth rate of the mutant was 0.094 g of DM liter-1 h-1, almost 6 times higher than the reported recombinant strain with XYL1 and XYL2 from P. stipitis. Additionally, the mutant showed a considerable xylose conversion that was 42% higher than the parental strain. Specific activities of XR, XDH and XK in the mutant displayed 2- to 5-fold of those in the original strain. Interestingly, the enzymatic activities in the glucose and xylose co-fermenting cells was up to 10-fold higher than in those grown solely on xylose. The DNA microarray results presented strong evidence that ethanol production in the mutant was mediated by endogenous xylose metabolism pathway with much higher transcriptional level of genes encoding XR, XDH and XK, genes involving in the pentose phosphate pathway and glycolytic metabolic pathway. More importantly, the glucose derepressible XR, XDH and XK of the mutant were also revealed by higher expression level of the relative genes in the glucose and xylose co-fermenting cells compared to the xylose growing cells. The results could be of significant importance in developing native xylose fermenting S.cerevisiae strain with innovate enzymatic design and modification of the endogenous XR, XDH and XK, as well as metabolic engineering of xylose metabolism pathway.