SUMMARY: Four mutant strains of , due to repeated mutation within the same short chromosome region, required arginine as a nutrient and failed to respond to citrulline. These strains accumulated argininosuccinic acid in the mycelium when grown on a medium supplemented with arginine. When the medium also contained citrulline this accumulation of argininosuccinic acid was enhanced, and citrulline also accumulated in the mycelium. Citrulline depressed the growth of the mutants at a concentration at which it had no effect on growth of the wild type.

Cell-free extracts of the wild type contained an argininosuccinase which catalysed the reversible splitting of argininosuccinic acid to arginine and fumaric acid. A procedure is described for the approximately fivefold purification of the enzyme. The pH optimum for the synthesis of argininosuccinic acid from arginine and fumarie acid was close to pH 7·0.

Neither cell-free extracts nor partially purified preparations from the four mutants showed any trace of argininosuecinase activity; 1-2%, and 0·2%, of normal wild-type activity should have been detectable in crude cell-free extracts and purified preparations, respectively. Neither the argininosuccinase activity of crude wild-type extracts, nor that of the partially purified preparations was appreciably altered by the addition of corresponding preparations from one of the mutants. A mixture of wild-type and mutant mycelium extracted together yielded the amount of activity to be expected on the assumption that the mutant mycelium was neither contributing to, nor decreasing, the yield.

Arginase and fumarase activities were similar in the wild type and one of the mutants.


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