Pyrimidine-Specific Carbamyl Phosphate Synthetase in Neurospora crassa

Abstract

Two carbamyl phosphate synthetases, the first an arginine-synthetic enzyme (CPS arg ) and the second a pyrimidine-synthetic enzyme (CPS pyr ), 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 CPS pyr and CPS arg have substrate requirements of adenosine triphosphate, HCO 3 − , and l -glutamine, although NH 4 + in high concentration will partially replace glutamine. CPS pyr activity can be completely inhibited by 5 × 10 −4 to 10 × 10 −4 m uridine triphosphate (UTP). CPS pyr is cold-labile and can be protected against cold inactivation by UTP. The synthesis of CPS pyr and aspartate transcarbamylase (ATC), the initial enzymatic steps of the pyrimidine pathway, are co-derepressed by pyrimidine starvation. Mutations affecting CPS pyr 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.