Sequence and regulation of psrA, a gene on the Sym plasmid of Rhizobium leguminosarum biover phaseoli which inhibits transcription of the psi genes.

The psr region of Rhizobium leguminosarum biovar phaseoli had originally been recognized on the basis of its ability to repress the transcription of the psi genes, one of which, psiA, inhibits exopolysaccharide synthesis when cloned in multi-copy plasmids. Both psr and psi are located on the symbiotic plasmid pRP2JI. The psrA gene was localized and sequenced. The deduced amino acid sequence of PsrA was shown to have similarity to the DNA-binding region of a family of other transcriptional regulators, consistent with its known effects on the expression of psi. The transcription of psrA itself appears to be constitutive in free-living Rhizobium, but is regulated by another gene on the Sym plasmid pRP2JI.