A method is described for isolating mutants of Saccharomyces cerevisiae which have lost repressibility by exogenous arginine for ornithine transcarbamylase.
Besides permeability mutants, three complementary classes of mutations were found: argRI, argRII and argRIII which are recessive and define three loci. No evidence for a linkage between any of these three loci or with the gene coding for ornithine transcarbamylase has been obtained.
Strains bearing mutations at either of these loci cannot be distinguished on a phenotype basis: after growth on minimal medium, the l-ornithine carbamoyl transferase activity is twice that of the wild type strain; the mutations modify neither the growth rate nor the permeability to arginine.
The mutations might affect the structure of an hetero-polypeptidic aporepressor.
The level of ornithine transcarbamylase in diploids is proportional to the number of argF+ genes in regulated as well as in non-regulated cells.