Transcription termination factor rho: the site of bicyclomycin inhibition in Escherichia coli

Abstract

Bicyclomycin is a novel, commercially important antibiotic. Information concerning the site of bicyclomycin inhibition in Escherichia coli has been obtained by the production of bicyclomycin resistant mutants by UV irradiation. Selection by growth in the presence of bicyclomycin of a plasmid clone library generated from a highly resistant mutant in recipient antibiotic-sensitive host cells (E. coli strain W3350) has led to the characterization of three different plasmids that confer drug resistance, which contained the gene encoding the transcription termination factor, rho. These mutant rho genes contained single base changes at nucleotide positions 656, 796, and 1009. Preliminary mechanistic information has been obtained by monitoring the polyC-dependent ATPase activity of rho in the absence and presence of bicyclomycin and dihydrobicyclomycin. Addition of bicyclomycin to aqueous solutions containing rho and ATP led to a decrease in the release of inorganic phosphate with an I50 value of 60-70 microM bicyclomycin. This inhibition is comparable to the drug concentration needed to inhibit bacterial growth on plates. No loss of activity was observed when a similar concentration of dihydrobicyclomycin was used in place of bicyclomycin, while use of 10-fold higher concentrations of this derivative led to partial rho inhibition. PolyC-dependent ATPase activity from partially purified rho isolated from the mutant BCMr108 was not inhibited by bicyclomycin at concentrations (200 microM) found to completely inhibit wild-type rho. These cumulative findings are consistent with the notion that bicyclomycin expresses its activity by interfering with the polyC-dependent ATPase activity of rho.