Genetics and physiology of proline utilization in Saccharomyces cerevisiae: mutation causing constitutive enzyme expression.

A mutation resulting in inducer-independent expression of the proline-degradative enzymes was isolated in the yeast Saccharomyces cerevisiae. Strains carrying the mutation, put3, are partially constitutive for proline oxidase and delta 1-pyrroline-5-carboxylate dehydrogenase when grown on a medium lacking proline and are hyperinducible for both enzyme activities when grown on a proline-containing medium. put3 segregates as a single nuclear gene, is not linked to either of the presumed structural genes for proline oxidase and delta 1-pyrroline-5-carboxylate dehydrogenase, and does not affect proline transport. When heterozygous in diploid strains, put3 behaves neither fully dominant nor fully recessive. Endogenous induction by proline has been eliminated as a cause of the inducer-independent enzyme expression in the put3 mutant and the mutation is believed to be in a regulatory component of the proline-degradative pathway.