Utilization of exogenous and endogenous ornithine by Neurospora crassa.

Through the use of a mutant deficient in ornithine-delta-transaminase (OTA), it is shown that this enzyme normally has no obligate or even major biosynthetic role in Neurospora. The pathways of ornithine and proline synthesis proceed wholly independently of each other in OTA-less strains. It is probable that OTA functions as an enzyme of arginine catabolism. With mutants affected in OTA, ornithine transcarbamylase, and the synthesis of ornithine, it was demonstrated that exogenous and endogenous ornithine are utilized in different ways. Exogenous ornithine is destined mainly for catabolism, whereas endogenous ornithine is destined mainly for biosynthesis. It is suggested that this distinction depends upon differences in the intracellular location or origin of the two sources of ornithine.