The transducing phage λ d supM814 and the plasmid pIB1830 containing the wild-type rpoB gene have been constructed and the primary structure of the gene's central fragment has been established. In contrast with the wild-type, the gene of the rpoB255 mutant, whose primary structure has been published, was found to contain an A.T.→T.A. transversion entailing the substitution of a valine residue for the aspartic acid residue (516) of the wild-type β subunit. © 1981 Springer-Verlag.