Glucose repression of lactose/galactose metabolism inKluyveromyces lactisis determined by the concentration of the transcriptional activator LA1C9 (K1GAL4)

In the budding yeast Kluyveromyces lactis glucose repression of genes involved in lactose and galactose metabolism is primarily mediated by LAC9 (or K1GAL4) the homologue of the well-known Saccharomyces cerevisiae transcriptional activator GAL4. Phenotypic difference in glucose repression existing between natural strains are due to differences in the LAC9 gene (Breunig, 1989, Mol.Gen.Genet. 261, 422-427). Comparison between the LAC9 alleles of repressible and non-repressible strains revealed that the phenotype is a result of differences in LAC9 gene expression. A two-basepair alteration in the LAC9 promoter region produces a promoter-down effect resulting in slightly reduced LAC9 protein levels under all growth conditions tested. In glucose/galactose medium any change in LAC9 expression drastically affects expression of LAC9 controlled genes e.g. those encoding beta-galactosidase or galactokinase revealing a strong dependence of the kinetics of induction on the LAC9 concentration. We propose that in tightly repressible strains the activator concentration drops below a critical threshold that is required for induction to occur. A model is presented to explain how small differences in activator levels are amplified to produce big changes in expression levels of metabolic genes.