Suppression of Amber and Ochre Mutants in Salmonella typhimurium by a Mutant F′-1-gal Factor Carrying an Ochre Suppressor Gene

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A Salmonella typhimurium strain was given the amber mutation hisC527 by transduction, made galactose-negative by mutation, then infected with the F′-1-gal factor. Of 107 spontaneous and mutagen-induced histidine-independent mutants tested, 3 proved to result from suppressor mutations within the F′ factor. The mutant F′ factors, when transferred to S. typhimurium and E. coli auxotrophs, suppressed amber and ochre but not UGA or missense mutants, and are inferred to carry ochre suppressor genes. Attempts to isolate an F′ amber suppressor mutant were unsuccessful. A suppressor F′ factor was transferred to 14 rough mutants which had been isolated from LT2 hisC527 (amber) by selection for resistance to phage P22.c2. One rough mutant was partly suppressed, as shown by its acquisition of O agglutinability and by alterations in its phage resistance pattern. Phage P22h grown on the suppressed mutant contransduced its rf. gene with cysE+ and with pyrE+, and the affected locus is inferred to be rfaL. Both the original and the mutant F′ factors conferred resistance to the rough-specific phage Br60, which is therefore “female-specific.”

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