Control of the ornithine cycle in Neurospora crassa by the mitochondrial membrane

In Neurospora crassa, the mitochondrial membrane separates ornithine used in arginine biosynthesis from ornithine used in the arginine degradative pathway in the cytosol. Ornithine easily exchanges across the mitochondrial membrane under conditions appropriate for synthesis of the immediate biosynthetic product, citrulline. Neither of the two mitochondrial enzymes required for the ornithine-to-citrulline conversion is feedback inhibitable in vitro. Nevertheless, when arginine is added to cells and cytosolic ornithine increases as arginine degradation begins, the rate of citrulline synthesis drops immediately to about 20% of normal (B. J. Bowman and R. H. Davis, Bacteriol. 130:285-291, 1977). We have studied this phenomenon in citrulline-accumulating strains carrying the arg-1 mutation. Citrulline accumulation is blocked when arginine is added to an arg-1 strain but not to an arg-1 strain carrying a mutation conferring insensitivity of intramitochondrial ornithine synthesis to arginine. Thus, ornithine is evidently unable to enter mitochondria in normal (feedback-sensitive) cells. Other experiments show that cytosolic ornithine enters mitochondria readily except when arginine or other basic amino acids are present at high levels in the cells. We conclude that in N. crassa, the mitochondrial membrane has evolved as a secondary site of feedback inhibition in arginine synthesis and that this prevents a wasteful cycling of catabolic ornithine back through the anabolic pathway. This is compared to the quite different mechanism by which the yeast Saccharomyces cerevisiae prevents a futile ornithine cycle.