STREPTOMYCIN- AND RIFAMPIN-RESISTANT MUTANTS OF ESCHERICHIA-COLI PERTURB F-EXCLUSION OF BACTERIOPHAGE-T7 BY AFFECTING SYNTHESIS OF THE F-PLASMID PROTEIN PIFA

Certain alleles of rpsL that confer resistance to the antibiotic strep tomycin almost completely relieve F exclusion of bacteriophage T7. Int roduction of a specific rpoB allele conferring resistance to rifampin into the rpsL strain restores the ability of the F-containing strain t o exclude T7. This variation in the severity of F exclusion is reflect ed in the levels of the F-encoded inhibitor protein PifA: F'-containin g cells that harbor specific rpsL alleles are phenotypically Pif(-), b ut become Pif(+) by the further acquisition of a specific rpoB allele. F-containing cells harboring the gyrA43(Ts) mutation also appear phen otypically Pif(-), possibly because repression of the pif operon is en hanced by an altered DNA conformation in the gyrase mutant strain.