The recovery of tryptophan A auxotrophs at high frequency in a strain ofSalmonella typhimurium

Abstract

1. Auxotrophs were sought in a slow-growing reversion of thetryA47strain ofS. typhimurium. This reversion differs fromtryA47by a genetic change that is inseparable from the47site and has been designated47S. Out of thirty-nine auxotrophs that derived from independent mutations, twenty-two grew on minimal medium supplemented with anthranilic acid. Eight of these auxotrophs were examined and each was shown to have the47Ssite unchanged and to carry a further mutation in thetryAgene. These further mutations were shown to be at different sites in different auxotrophs.2. Auxotrophs were sought in the wild-type LT-2 strain. None out of thirty-six were mutant in thetryAgene.3. The47Ssite intryA47Swas replaced by47+transduced from wild-type LT-2. Auxotrophs were sought in this strain and only one out of nineteen was mutant in thetryAgene.4. Auxotrophs were sought in a wild-type reversion oftryA47. Out of twenty-six none were mutant in thetryAgene.5. The47+site in wild-type LT-2 was replaced by47Stransduced fromtryA47S. Auxotrophs were sought in this strain and twenty-two out of fifty-five were mutant in thetryAgene.6. We conclude thattryAauxotrophs are only recovered at a high frequency when the47Ssite is present in thetryAgene.7. In strains with the47Ssite in thetryAgene the frequency of auxotrophs that will grow on minimal medium supplemented with indole but not on minimal medium supplemented with anthranilic acid is appreciably higher than in those strains without the47Ssite in thetryAgene. These auxotrophs are mutant in genes that are in the same operon astryA.