CONSTITUTIVE SYNTHESIS OF THE GAL4 PROTEIN, A GALACTOSE PATHWAY REGULATOR IN SACCHAROMYCES CEREVISIAE

We have demonstrated that induction of synthesis of the mRNAs encoding the galactose-catabolizing enzymes galactokinase (E.C.2.7.1.6) and transferase (E.C.2.7.7.12) occurs in Saccharomyces cerevisiae following the addition of α-D-galactose during continuous exposure to 100 μg/ml of cycloheximide. The prompt induction and sustained accumulation of galactose message activity (detected in the wheat germ in vitro translation system) following the cessation of all detectable protein synthesis in vivo indicate that the GAL4 regulatory protein and any as yet undefined positive effectors of transcription of the galactose genes exist prior to induction and are therefore synthesized constitutively. We have carried out synchronous matings in which inducible gal1 cells lacking galactokinase function were mated with uninducible gal4 cells containing either the normal GAL80-encoded galactose-sensitive repressor or a dominant GAL80S-encoded galactose-insensitive repressor. All cells were cultured in the continuous presence of galactose to allow for the possibility of inducible synthesis of the GAL4-specified protein. Galactokinase was expressed in the GAL80/GAL80 zygote (the control experiment) and not in the GAL80S/GAL80 zygote, indicating that a galactose-insensitive repressor can neutralize already existing GAL4 protein activity. These results, particularly the cycloheximide finding, necessitate a revision of a central element of the Douglas and Hawthorne (1966) model for galactose regulation, in which it was postulated that the GAL80 gene product acts at a GAL4 operator to repress GAL4 transcription. Our observations indicate that the repressor must control GAL4 regulatory activity at a post-translational level. © 1979.