Streptolydigin resistance can be conferred by alterations to either the beta or beta' subunits of Bacillus subtilis RNA polymerase

Abstract

Rifampicin and streptolydigin are antibiotics which inhibit prokaryotic RNA polymerase at the initiation and elongation steps, respectively. In Escherichia coli, resistance to each antibiotic results from alterations in the beta subunit of the core enzyme. However, in Bacillus subtilis, reconstitution studies found rifampicin resistance (RifR) associated with the beta subunit and streptolydigin resistance (StlR) with beta'. To understand the basis of bacterial StlR, we isolated the B. subtilis rpoC gene, which encodes a 1,199-residue product that is 53% identical to E. coli beta'. Two spontaneous StlR mutants carried the same D796G substitution in rpoC, and this substitution alone was sufficient to confer StlR in vivo. D796 falls within Region F, which is conserved among the largest subunits of prokaryotic and eukaryotic RNA polymerases. Among eukaryotes, alterations in Region F promote resistance to alpha-amanitin, a toxin which inhibits transcription elongation; among prokaryotes, alterations in Region F cause aberrant termination. To determine whether alterations in the beta subunit of B. subtilis could also confer StlR, we made three StlR substitutions (A499V, G500R, and E502V) in the rif region of rpoB. Together these results suggest that beta and beta' interact to form an Stl binding site, and that this site is important for transcription elongation.