A mutant form of ornithine transcarbamylase found in a strain of Neurospora carrying a pyrimidine-proline suppressor gene.

A mutation, s, of Neurospora, leads to a 98% reduction in ornithine transcarbamylase (OTC) activity without imposing an arginine requirement. It has been found that the enzyme present in the s strain is different from the normal enzyme in its affinities for ornithine and carbamyl phosphate. The affinity of the mutant enzyme for carbamyl phosphate is much greater than in the case of the wild type enzyme, and this characteristic could compensate to a large extent for the low specific activity in s strains. The data are used to indicate that the s locus is a gene which controls the structure of OTC in Neurospora. The possibility of there being two forms of this enzyme in wild type strains is discussed.