Cellular distribution of ornithine in Neurospora: anabolic and catabolic steady states.

During growth on minimal medium, cells of Neurospora contain three pools of ornithine. Over 95% of the ornithine is in a metabolically inactive pool in vesicles, about 1% is in the cytosol, and about 3% is in the mitochondria. By using a ureaseless strain, we measured the rapid flux of ornithine across the membrane boundaries of these pools. High levels of ornithine and the catabolic enzyme ornithine aminotransferase coexist during growth on minimal medium but, due to the compartmentation of the ornithine, only 11% was catabolized. Most of the ornithine was used for the synthesis of arginine. Upon the addition of arginine to the medium, ornithine was produced catabolically via the enzyme arginasn early enzyme of ornithine synthesis. The biosynthesis of arginine itself, from ornithine and carbamyl phosphate, was halted after about three generations of growth on arginine via the repression of carbamyl phosphate synthetase A. The catabolism of arginine produced ornithine at a greater rate than it had been produced biosynthetically, but this ornithine was not stored; rather it was catabolized in turn to yield intermediates of the proline pathway. Thus, compartmentation, feedback inhibition, and genetic repression all play a role to minimize the simultaneous operation of anabolic and catabolic pathways for ornithine and arginine.