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 lleles) 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 lleles affect ATC via different mechanisms.
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