XYLOSE-METABOLIZING SACCHAROMYCES-CEREVISIAE STRAINS OVEREXPRESSING THE TKL1 AND TALI GENES ENCODING THE PENTOSE-PHOSPHATE PATHWAY ENZYMES TRANSKETOLASE AND TRANSALDOLASE

Saccharomrces cerevisiae was metabolicaliy engineered for xylose utili zation. The Pichia stipitis CBS 6054 genes XYL1 and XYL2 encoding xylo se reductase and xylitol dehydrogenase were cloned into S. cerevisiae. The gene products catalyze the two initial steps in xylose utilizatio n which S. cerevisiae lacks. In order to increase the flux through the pentose phosphate pathway, the S. cerevisiae TKL1 and TAL1 genes enco ding transketolase and transaldolase were overexpressed. A XYL1- and X YL2-containing S. cerevisiae strain overexpressing TAL1(S104-TAL) show ed considerably enhanced growth on xylose compared with a strain conta ining only XYL1 and XYL2. Overexpression of only TKL1 did not influenc e growth. The results indicate that the transaldolase level in S. cere visiae is insufficient for the efficient utilization of pentose phosph ate pathway metabolites. Mixtures of xylose and glucose were simultane ously consumed with the recombinant strain S104-TAL. The rate of xylos e consumption was higher in the presence of glucose. xylose was used f or growth and xylitol formation, but not for ethanol production. Decre ased oxygenation resulted in impaired growth and increased xylitol for mation. Fermentation with strain S103-TAL, having a xylose reductase/x ylitol dehydrogenase ratio of 0.5:30 compared with 4.2:5.8 for S104-TA L, did not prevent xylitol formation.