[Mutations in the Escherichia coli RNA-polymerase beta-subunit gene cloned in a multicopy plasmid].

A multicopy plasmid pLMN1 expressing a wild type rpoB gene encoding Escherichia coli RNA polymerase beta subunit gene was constructed. Introduction of this plasmid into rifampicin-resistant RpoB mutants makes them rifampicin-sensitive. Rifampicin-resistant clones appear in such strains with frequencies up to 10(-3), due to recombinational (recA-dependent) transfer of rif-r mutations from chromosome to pLMN1. This provides a simple selection procedure for transfer of any rpoB mutation, together with a rif-r mutation from a chromosome to pLMN1. In this way, we transferred rpoB22 amber mutation to pLMN1 for localization of the mutant codon by DNA sequencing.