Pyrimidine-Specific Carbamyl Phosphate Synthetase in Neurospora crassa

Two carbamyl phosphate synthetases, the first an arginine-synthetic enzyme (CPSarg ) and the second a pyrimidine-synthetic enzyme (CPSpyr ), are shown to be present inNeurospora . 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 − , andl -glutamine, although NH4 + in high concentration will partially replace glutamine. CPSpyr activity can be completely inhibited by 5 × 10−4 to 10 × 10−4 m 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.