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Background  Overexpression of the  PGM2  gene encoding phosphoglucomutase (Pgm2p) has been shown to improve galactose utilization both under aerobic and under anaerobic conditions. Similarly, xylose utilization has been improved by overexpression of genes encoding xylulokinase (XK), enzymes from the non-oxidative pentose phosphate pathway (non-ox PPP) and deletion of the endogenous aldose reductase  GRE3  gene in engineered  Saccharomyces cerevisiae  strains carrying either fungal or bacterial xylose pathways. In the present study, we investigated how the combination of these traits affect xylose and galactose utilization in the presence or absence of glucose in  S. cerevisiae  strains engineered with the xylose reductase (XR)-xylitol dehydrogenase (XDH) pathway.    Results  In the absence of  PGM2  overexpression, the combined overexpression of XK, the non-ox PPP and deletion of the  GRE3  gene significantly delayed aerobic growth on galactose, whereas no difference was observed between the control strain and the xylose-engineered strain when the  PGM2  gene was overexpressed. Under anaerobic conditions, the overexpression of the  PGM2  gene increased the ethanol yield and the xylose consumption rate in medium containing xylose as the only carbon source. The possibility of Pgm2p acting as a xylose isomerase (XI) could be excluded by measuring the XI activity in both strains. The additional copy of the  PGM2  gene also resulted in a shorter fermentation time during the co-consumption of galactose and xylose. However, the effect was lost upon addition of glucose to the growth medium.    Conclusions   PGM2  overexpression was shown to benefit xylose and galactose fermentation, alone and in combination. In contrast, galactose fermentation was impaired in the engineered xylose-utilizing strain harbouring extra copies of the non-ox PPP genes and a deletion of the  GRE3  gene, unless  PGM2  was overexpressed. These cross-reactions are of particular relevance for the fermentation of mixed sugars from lignocellulosic feedstock.

Cross-reactions between engineered xylose and galactose pathways in recombinant Saccharomyces cerevisiae