Nucleic Acids Research, 1993, Vol. 21, No. 1 69-77
© 1993
MOLECULAR BIOLOGY |
Glucose repression of lactose/galactose metabolism in Kluyveromyces lactis is determined by the concentration of the transcriptional activator LA1C9 (K1GAL4)
Institut für Mikrobiologie, Heinrich-Heine-Universität Düsseldorf 4000 Düsseldorf, Germany
*To whom correspondence should be addressed
Received September 28, 1992. Accepted December 3, 1992.
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 represslble and non-represslble 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 ß-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.
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