Neurospora crassa Mutants Lacking Argininosuccinase

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Abstract
Four strains, 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 catalyzed the reversible splitting of argininosuccinic acid to arginine and fumaric acid. Neither cell-free extracts nor partially purified preparations from the 4 mutants showed any trace of argininosuccinase activity. The investigation appears to support the one gene-one enzyme hypothesis.