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

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.