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Pyrimidine-Specific Carbamyl Phosphate Synthetase in Neurospora crassa

¹ Department of Botany, The University of Michigan, Ann Arbor, Michigan 48104

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, HCO₃^–, and L-glutamine, although NH₄⁺ in high concentration will partially replace glutamine. CPS_pyr activity can be completely inhibited by 5 x 10^–4 to 10 x 10^–4M 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.

This article has been cited by other articles:

• Davis, R. H. (1972). Metabolite Distribution in Cells: Two carbamyl phosphate gradients and their sources can be discerned in Neurospora. Science 178: 835-840