Suppression of proline requirement of proA and proAB deletion mutants in Salmonella typhimurium by mutation to arginine requirement.

Eleven variants able to grow without proline (provided arginine was absent) were obtained by spontaneous mutation from Salmonella typhimurium LT7 proA and proAB deletion mutants. Suppression resulted from mutation at argG, which specifies N(alpha)-acetylornithine delta-transaminase. In the absence of exogenous arginine, deficiency of this enzyme would cause derepression of the arginine pathway and accumulation of N-acetylglutamic gamma-semialdehyde. N-acetylglutamic gamma-semialdehyde, if deacetylated, would produce glutamic gamma-semialdehyde, the proline precursor whose synthesis from glutamate is blocked in proA and proAB mutants. All of the mutants grew only slowly (some very slowly) if not supplied with arginine. Sonic-treated preparations of eight mutants had no measurable acetylornithine delta-transaminase activity, but those of the three mutants least dependent on arginine had 0.11, 0.28, and 1.48 of wild-type activity; presumably, their enzymes have low specific activity, at least in vivo. Phage P22 cotransduced argG and strA. Genetic analysis showed that the minor degree of arginine dependence of the mutant with greater than wild-type in vitro enzyme activity was a characteristic of its argG allele, not the result of modification of the argG phenotype by mutation elsewhere.