A genetic study of primary and secondary reversions of sometryptophanAauxotrophs ofSalmonella typhimurium

Abstract

1. The linkage order of fourtryAmutants ofS. typhimurium, andcysB-12, is:Attempts to plot the position oftryA-50were unsuccessful.2. Some of the reversions oftryA-8,tryA-47,tryA-56andtryA-50were analysed genetically;tryA-52does not revert. All four auxotrophs gave reversions that were phenotypically and genetically indistinguishable from that expected by back-mutation of the original mutant site.3. BothtryA-8andtryA-50produced reversions that grew as wild-type but were due to unlinked suppressor mutations. Some of these were super-suppressors in that they suppressed bothtryA-8andtryB-4; others suppressed many site mutants in thetryAgene but did not suppresstryB-4.4. All the slow-growing reversions oftryA-8,tryA-50andtryA-56, and a minority of the semi-fast reversions oftryA-8, were due to unlinked suppressors.5. All the slow-growing reversions oftryA-47, the semi-fast reversions oftryA-56and the majority of the semi-fast reversions oftryA-8were due to genetic changes that were inseparable, in very extensive experiments, from their original mutant site.6. Slow-growing reversions oftryA-47produced faster growing mutants. Some of these were due to mutation in unlinked modifying genes and in others the genetic change was within thetryAgene. Nine of the latter had the genetic change just to the left of the47Ssite; in one the change was inseparable from the47Ssite. None had this further change to the right of the47Ssite. These further changes, in the absence of the47Ssite, gave prototrophic phenotypes; they are inter-site suppressors.