Pyrimidine-Specific Carbamyl Phosphate Synthetase in Neurospora crassa

AUTOR(ES)

Williams, Larry G.

RESUMO

Two carbamyl phosphate synthetases, the first an arginine-synthetic enzyme (CPSarg) and the second a pyrimidine-synthetic enzyme (CPSpyr), are shown to be present in Neurospora. The two enzymes can be separated on the basis of size and are distinguished by several different properties. Both CPSpyr and CPSarg have substrate requirements of adenosine triphosphate, HCO3−, and l-glutamine, although NH4+ in high concentration will partially replace glutamine. CPSpyr activity can be completely inhibited by 5 × 10−4 to 10 × 10−4m uridine triphosphate (UTP). CPSpyr is cold-labile and can be protected against cold inactivation by UTP. The synthesis of CPSpyr and aspartate transcarbamylase (ATC), the initial enzymatic steps of the pyrimidine pathway, are co-derepressed by pyrimidine starvation. Mutations affecting CPSpyr and ATC all map at the same locus, pyr-3. Three classes of mutants with respect to the two activities were found: CPS+ATC−, CPS−ATC+, and CPS−ATC−. The distribution of these mutants on the genetic map, together with other data, indicate that the two activities are carried by a bifunctional protein.

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