The nucleotide sequence of the rpoB gene of Salmonella typhimurium has been determined in this work. It was compared with known sequences of the gene from other sources and the conservative regions were detected. This allowed some interesting conclusions to be made about the distribution of the functional domains in bacterial RNA polymerase and about the three-dimensional structure of its p subunit.DNA-dependent synthesis of all types of RNA in bacteria is catalyzed by RNA polymerase, an oligomeric enzyme with the subunit structure .$j?'o (for review see [l]). To study the structure and the mechanism of RNA polymerase action, amino acid sequences of all subunits of the E. coli enzyme were determined [2-51, the subunit in contact with the DNA template, RNA product and substrates were identified [6] and several mutations leading to RNA polymerase resistance to the antibiotics rifampicin and streptolydigin were localized [7 -101. These investigations indicated an important role for the RNA polymerase p subunit at all stages of RNA synthesis. This paper presents the results of cloning and sequencing of the rpoB gene coding for the subunit of Salmonella typhimurium RNA polymerase.The primary structure of the Salmonella typhimurium rpoB gene coding for RNA polymerase p subunit was determined (4.2 kb in toto). On comparison with homologous Escherichia coli and tobacco chloroplasts gene sequences, nine conservative and six variable regions of the [ j subunit structure were identified. Analysis of available genetic data allowed some preliminary conclusions to be made concerning the location of the putative functional domains in the structure of the p subunit of bacterial RNA polymerase and to devise a hypothesis about the spatial proximity of the central part of the p subunit to its N-and C-terminal domains.
The unusual recombinant plasmid pRC19 carrying the N-terminal fragment of the Escherichia coli RNA polymerase rpoB gene was found to specify high level rifampicin resistance of E. coli cells. Sequence analysis of this plasmid revealed one substitution only: transversion G----T, leading to amino acid substitution Val146----Phe. This mutational change marks the second domain of the beta subunit involved in rifampicin binding.
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