Abstract
About 200 forward mutants of Neurospora obtained as suppressors of arg were studied. All fell in the pyr region, and all appeared deficient in aspartic transcarbamylase (ATC) to various degrees: extreme deficiencies in ATC (pyr-N alleles) resulted in pyrimidine-dependence as well as suppression of arg, while partial ATC deficiencies (pyrsu-arg alleles) led only to suppression. Five different complementation groups were represented among the pyr-N alleles studied. Evidence is presented which suggests that pyr-N and pyrsu-arg alleles affect ATC via different mechanisms.