Arginine Auxotrophic Phenotype of Mutations in pyrA of Salmonella typhimurium : Role of N -Acetylornithine in the Maturation of Mutant Carbamylphosphate Synthetase

Mutations inpyrA that abolish catalytic activity of carbamylphosphate synthetase cause auxotrophy for both arginine and a pyrimidine. EightpyrA mutants auxotrophic only for arginine (AUX) were isolated by the mutagenized phage technique; three of these required arginine only at low temperature (20°C). Explanations of the AUX phenotype based on bradytrophy were eliminated by the discovery that blocking the utilization of carbamylphosphate for pyrimidine biosynthesis by insertion of an additional mutation inpyrB (encoding aspartic transcarbamylase) did not reduce the requirement for arginine. In contrast, mutational blocks in the arginine biosynthetic pathway beforeN -acetylornithine (argB, argC, argG , orargH ) did suppress the mutation inpyrA . This suggests that exogenous arginine permits growth of the AUX mutants by inhibiting the first step in the arginine pathway, thereby preventing accumulation of an intermediate that antagonizes mutantpyrA function. A mutation inargA (N -acetylornithinase) failed to suppress AUX, indicating thatN -acetylornithine was the inhibitory intermediate. This intermediate had no effect on the catalytic or regulatory properties of carbamylphosphate synthetase from mutant cells grown under permissive conditions (37°C). However, the regulatory properties of carbamylphosphate synthetase synthesized under restrictive conditions (20°C) were demonstrably defective (insensitive to activation by ornithine); the enzyme synthesized under permissive conditions was activated by ornithine. A strain carrying an additional mutation (argC ), which prevents the accumulation ofN -acetylornithine, produced an ornithine-activatable enzyme at both growth temperatures. These results suggest thatN -acetylornithine antagonizes the proper preconditioning or maturation of the mutant carbamylphosphate synthetase.