Cyclic AMP may not be involved in catabolite repression in Saccharomyes cerevisiae: evidence from mutants capable of utilizing it as an adenine source.

Mutants able to utilize 5'-AMP or cyclic AMP as the adenine source were isolated from an ade6 ade10 double mutant by ethyl methane sulfonate mutagenesis. A single amp1 mutation, primarily selected on 5'-AMP medium, confers the phenotype for utilization of exogenous 5'-AMP as the adenine source. From the ade6 ade10 amp1 triple mutant, a mutant able to utilize cyclic AMP was isolated, and the mutant phenotype was proven to be due to the simultaneous occurrence of triple mutations designated as cam1, cam2, and cam3. The cam3 mutation, but not cam1 or cam2, also confers the phenotype for utilizing 5'-AMP, the same phenotype as the amp1 mutation. All of these mutations are recessive to the respective wild-type counterparts. Cells having the ade6 ade10 amp1 cam1 cam2 cam3 genotype showed significant ability to take up exogenous cyclic AMP, whereas no differences were observed in cyclic AMP phosphodiesterase activity in comparison with that of the original strains used in the mutant isolation. Since glucose severely repressed galactokinase synthesis in the constitutive GAL81 mutant having the ade6 ade10 amp1 cam1 cam2 cam3 genotype, irrespective of the presence or absence of cyclic AMP in the medium, it was suggested that cyclic AMP is not involved in the mechanism of catabolite repression in Saccharomyces cerevisiae. It does, however, have a stimulative effect on the galactokinase synthesis in the GAL81 mutant in the absence of glucose.