Cloning of hexokinase isoenzyme PI from Saccharomyces cerevisiae: PI transformants confirm the unique role of hexokinase isoenzyme PII for glucose repression in yeasts.

Hexokinase isoenzyme PI was cloned using a gene pool obtained from a yeast strain having only one functional hexokinase, isoenzyme PI. The gene was characterized using 20 restriction enzymes and located within a region of 2.0 kbp. The PI plasmid strongly hybridized with the PII plasmids isolated previously (Fröhlich et al. 1984). Hence there was a close relationship between the two genes, one of which must have been derived from the other by gene duplication. In contrast, glucose repression was restored only in hexokinase PII transformants; PI transformants remained non-repressible. This observation provided additional evidence for the hypothesis of Entian (1980) that only hexokinase PII is necessary for glucose repression. Furthermore, glucose phosphorylating activity in PI transformants exceeded that of wild-type cells, giving clear evidence that the phosphorylating capacity is not important for glucose repression.