Genes influencing the conversion of citrulline to argininosuccinate in Neurospora crassa.

The enzymic conversion of citrulline + argininosuccinate + arginine in wild-type Neurospora crasm has been shown to be essentially like that in mammalian tissues in all respects tested. The enzymes responsible for the two reactions (condensing enzyme and argjninosuccinase) have been partially separated. Five mutants a t the arg-1 locus have normal argininosuccinase and little or no condensing activity. The lack of condensing activity appears to be due to a simple absence of enzyme, alternatives such as inhibitor production or increased ATPase competition having been ruled out. Small amounts of apparent condensing activity, detected in the substrate-disappearance assay, were shown to be due to side reactions. When grown a t the usual high arginine concentrations, three mutants at the arg-10 locus, which are known to lack argininosuccinase, have normal condensing activity. However, when arg-10 strains are grown a t low arginine concentrations, the resulting extracts have very little condensing activity. This also appears to be due to a simple loss of condensing enzyme. It has not been determined whether the low activity is a specific secondary effect of arg-10 on the condensing enzyme, or whether i t is merely a non-specific effect of the inadequate supplementation, which might cause reductions in many enzyme activities. Despite its very low condensing activity in zritro, arg-10 grown a t low arginine concentrations must be active at some time in vivo, since its mycelium accumulates argininosuccinate but not citrulline. In contrast, an arg-1 arg-10 double mutant, grown a t low arginine concentrations, must be inactive iu vivo, since it accumulates citrulline but not argininosuccinate. It is concluded that arg-1 is probably the primary locus controlling the synthesis of condensing enzyme.