Consequences of a suppressor gene effective with pyrimidine and proline mutants of Neurospora.

HIS study was undertaken to find a common factor responsible for the various effects of a suppressor mutation, s, which is able to eliminate the nutritional requirement of certain mutants at the pyrimidine-3 locus and of those at the proline-2 and -3 loci of Neurospora (HOULAHAN and MITCHELL 1947; MITCHELL and MITCHELL 1952). The s gene was not detectable, however, aside from its interaction with other mutations. Because the action of the suppressor with regard to pyr-3a can be reversed by the addition of arginine to cultures of the suppressed mutant, pyr-3a-s (HOULAHAN and MITCHELL 1947), it was felt that strains carrying pyr-3a were abnormally sensitive to arginine, and that the s gene led to a deficiency in the synthesis of arginine (DAVIS 1961). Experiments involving the enzyme ornithine transcarbamylase (OTC) are described here which support this view in part. The enzyme catalyzes the carbamylation of ornithine by carbamyl phosphate (CAP) to form citrulline (an arginine precursor) with the liberation of inorganic phosphate. The results are discussed with particular attention to previously described interactions of s with proline, arginine, and pyrimidine mutants. A preliminary report on this work has been published previously (DAVIS 196 1 ) .