Mutations in the β subunit of the Bacillus subtilis RNA polymerase that confer both rifampicin resistance and hypersensitivity to NusG

Mutations conferring resistance to the antibiotic rifampicin (Rif super(r)) occur at specific sites within the beta subunit of the prokaryotic RNA polymerase. Rif super(r) mutants of Escherichia coli are frequently altered in the elongation and termination of transcription. Rif super(r) rpoB mutations were isolated in Bacillus subtilis and their effects on transcription elongation factor NusG and Rho-dependent termination were investigated. RNase protection assay, Northern analysis and the expression of nusG-lacZ fusions in cells with an inducible NusG suggested the B. subtilis nusG gene was autoregulated at the level of transcription. Rif super(r) mutations that changed residue Q469 to a basic residue (Q469K and Q469R) enhanced autoregulation of nusG. A mutant expressing a truncated form of NusG, due to a nonsense mutation within the nusG gene, was isolated on the basis of the loss of autoregulation. The mechanism of autoregulation was found to be independent both of transcription termination factor Rho and of the promoter transcribing nusG. Autoregulation required sequences within the 5' coding sequence of the nusG gene or immediately upstream. This is the first evidence from any bacterium that Rif super(r) RNA polymerases can display altered transcription regulation by NusG.