Possible role of a regulatory gene product upon the myo-inositol-1-phosphate synthase production in Neurospora crassa

The regulatory effect of inositol on inositol-1-phosphate synthase in Neurospora crassa strains was studied. Inositol represses enzyme production in the cultures of the wild type and that of the thermosensitive inositol-requiring mutant grown at 22°C. Enzyme activity as well as the quantity of enzyme protein decreased sharply in both strains by increasing concentrations of inositol in the medium. Inositol-requiring strains used in our experiments can be divided into two groups. The first group produces a protein related immunologically to inositol phosphate synthase, but which is enzymatically inactive. The synthesis of this defective enzyme was also repressed by inositol. In the second group, this protein was found to be completely lacking, in both the thermosensitive mutant grown at 37°C, and in a strain requiring inositol due to a reciprocal translocation. The thermostability and the cross immunoelectrophoresis of the enzyme suggest that in the case of the thermosensitive inositol-requiring mutant, the mutation did not occur in the structural gene of the enzyme, but its regulation was probably affected.