EXPRESSION OF DIFFERENT LEVELS OF ENZYMES FROM THE PICHIA-STIPITIS XYL1 AND XYL2 GENES IN SACCHAROMYCES-CEREVISIAE AND ITS EFFECTS ON PRODUCT FORMATION DURING XYLOSE UTILIZATION

Saccharomyces cerevisiae was transformed with the Pichia stipitis CBS 6054 XYL1 and XYL2 genes encoding xylose reductase (XR) and xylitol de hydrogenase (XDH) respectively. The XYL1 and XYL2 genes were placed un der the control of the alcohol dehydrogenase 1 (ADH1) and phosphoglyce rate kinase (PGK1) promoters in the yeast vector YEp24. Different vect or constructions were made resulting in different specific activities of XR and XDH. The XR:XDH ratio (ratio of specific enzyme activities) of the transformed S. cerevisiae strains varied from 17.5 to 0.06. In order to enhance xylose utilisation in the XYL1-, XYL2-containing S. c erevisiae strains, the native genes encoding trans-ketolase and transa ldolase were also overexpressed. A strain with an XR:XDH ratio of 17.5 formed 0.82 g xylitol/g consumed xylose, whereas a strain with an XR: XDH ratio of 5.0 formed 0.58 g xylitol/g xylose. The strain with an XR :XDH ratio of 0.06, on the other hand, formed no xylitol and less glyc erol and acetic acid compared with strains with the higher XR:XDH rati os. In addition, the strain with an XR:XDH ratio of 0.06 produced more ethanol than the other strains.