J. Biol. Chem., Vol. 262, Issue 15, 7109-7117, 05, 1987
Arginine-specific carbamoyl phosphate metabolism in mitochondria of Neurospora crassa. Channeling and control by arginine
RH Davis and JL Ristow
Citrulline is synthesized in mitochondria of Neurospora crassa from
ornithine and carbamoyl phosphate. In mycelia grown in minimal medium,
carbamoyl phosphate limits citrulline (and arginine) synthesis. Addition of
arginine to such cultures reduces the availability of intramitochondrial
ornithine, and ornithine then limits citrulline synthesis. We have found
that for some time after addition of excess arginine, carbamoyl phosphate
synthesis continued. Very little of this carbamoyl phosphate escaped the
mitochondrion to be used in the pyrimidine pathway in the nucleus. Instead,
mitochondrial carbamoyl phosphate accumulated over 40-fold and turned over
rapidly. This was true in ornithine- or ornithine
carbamoyltransferase-deficient mutants and in normal mycelia during
feedback inhibition of ornithine synthesis. The data suggest that the rate
of carbamoyl phosphate synthesis is dependent to a large extent upon the
specific activity of the slowly and incompletely repressible synthetic
enzyme, carbamoyl- phosphate synthetase A. In keeping with this conclusion,
we found that when carbamoyl-phosphate synthetase A was repressed 2-10-fold
by growth of mycelia in arginine, carbamoyl phosphate was still synthesized
in excess of that used for residual citrulline synthesis. Again, only a
small fraction of the excess carbamoyl phosphate could be accounted for by
diversion to the pyrimidine pathway. The continued synthesis and turnover
of carbamoyl phosphate in mitochondria of arginine-grown cells may allow
rapid resumption of citrulline formation after external arginine disappears
and no longer exerts negative control on ornithine biosynthesis.
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