HEXOKINASE-ACTIVITY IS AFFECTED IN MUTANTS OF KLUYVEROMYCES-LACTIS RESISTANT TO GLUCOSE REPRESSION

The effect of glucose on a number of mitochondrial and cytoplasmic enzymes involved in carbon metabolism (L-lactate:ferricytochrome-c 2-oxidoreductase, malate dehydrogenase, beta-galactosidase, invertase, maltase and NAD-glutamate dehydrogenase) has been analysed in two different strains of Kluyveromyces lactis (PM4-4B and JA6 strains). All the above mentioned enzymes were catabolite-repressible in a strain-dependent way. From this study differences in the regulation of some enzymes have been observed between K. lactis and Saccharomyces cerevisiae. To identify genes involved in glucose metabolism that may also control carbon catabolite repression, 2-deoxyglucose-resistant mutants (Dgr(+)) of K. lactis that were unable to grow on glucose in the presence of antimycin A (Rag(-)) were selected. In this way we identified four classes of mutants. Two of them define genes previously identified: RAG1 and RAG5, encoding the low affinity glucose transport system and hexokinase, respectively. The two remaining classes of mutants define two new genes, DGR148 and DGR239, that control the level of hexokinase, indicating that in K. lactis this enzyme is positively regulated by at least two genes. All the mutants devoid of hexokinase showed relief from carbon catabolite repression of several enzymes.