Genetic evidence for the interaction between cluster I and cluster III rifampicin resistant mutations

Rifampicin-resistant (Rifr) mutations of Escherichia coli map to the central portion of the rpoB gene, which encodes the beta subunit of RNA polymerase. These mutations are located in three distinct clusters, designated I, II and III. Three intragenic suppressors of the cluster III Rifr mutation, rpoB3406(RH687), restore the ability of the mutant strain to grow at low and high temperatures and map to a single locus in cluster I. These suppressors are identical to two previously characterized Rifr alleles, rpoB3401(RC529) and rpoB3402(RS529). None of the other 14 previously identified Rifr mutations that we have characterized confers this phenotype. We suggest that this allele-specific suppression results from interaction between Cluster I and Cluster III of the beta subunit.