CARBON CATABOLITE REGULATION OF TRANSCRIPTION OF NUCLEAR GENES-CODINGFOR MITOCHONDRIAL PROTEINS IN THE YEAST KLUYVEROMYCES-LACTIS

Promoter regions of the KlQCR7, KlQCR8 and KlCYC1 genes, coding for su bunits of the bc(1)-complex and cytochrome c respectively, in the shor tterm Crabtree-negative yeast Kluyveromyces lactis differ markedly in sequence from their Saccharomyces cerevisiae counterparts. They have, however, conserved very similar configurations of binding-site motifs for various transcription factors known to be involved in global and c arbon-source regulation in S. cerevisiae. To investigate the carbon so urce-dependent expression of these genes in K. lactis, we have carried out medium-shift experiments and determined transcript levels during the shifts. In sharp contrast to the situation in S. cerevisiae, the l evel of expression in K. lactis is not affected when glucose is added to a non-fermentable carbon-source medium. However, the genes are not constitutively expressed, but become significantly induced when the ce lls are shifted from glucose to a nonfermentable carbon source. Finall y, induction of transcriptional activation does not occur in media con taining both glucose and non-fermentable carbon sources.