Characterization of a yeast regulatory mutant constitutive for synthesis of inositol-1-phosphate synthase.

The enzyme inositol-1-phosphate synthase is repressed at least 50-fold in wild type yeast grown in inositol-supplemented media. Mutants which synthesize this enzyme constitutively have been isolated using a selection procedure based on excretion of inositol into the growth medium by putative mutants. Biochemical analysis of one of the mutants (opi1-1) confirmed that the nature of the mutations is regulatory, and not in the structural gene for the enzyme. Immunoprecipitation of crude extracts with antibody directed against purified inositol-1-phosphate synthase showed that a protein which reacts with the antibody is present in the mutant grown under both repressing and derepressing conditions, in contrast to the wild type which synthesizes the enzyme only when derepressed. Assay of inositol-1-phosphate synthase activity in crude extracts of the mutant verified synthase activity in cells grown under both repressing and derepressing conditions. Synthase purified from this mutant was characterized with respect to molecular weight, thermolability and affinity for substrates glucose-6-phosphate and NAD. These analyses indicated that purified mutant synthase was similar to the wild type enzyme.