Genetic studies on the beta subunit of Escherichia coli RNA polymerase. III. Analysis of low-level rifampicin-resistant mutants.

ABSTRACT

Spontaneously-occurring rifampicin-resistant mutants that survive on low (20 micrograms ml-1) but not high drug concentrations (200 micrograms ml-1) have been isolated. One such mutation appears to map close to residue 650 on the beta structural gene. RNA polymerase from low-level resistant strains shows an intermediate response to rifampicin in vitro both the association and dissociation rates of the antibiotic are markedly reduced. Continued expression of this unusual phenotype upon introduction of a high-level resistance allele is described in terms of DNA blockade.