Two genes encoding a putative multidrug efflux pump of the RND/MFP family are cotranscribed with an rpoH gene in Bradyrhizobium japonicum

The rpoH 3 gene of Bradyrhizobium japonicum codes for one of three σ 32-type transcription factors in this organism and is flanked by rag ( r poH 3 - a ssociated) g enes comprising the chromosomal arrangement ragABrpoH 3ragCD . The first genes in this cluster code for a classical two-component regulatory system with an unknown function (Narberhaus et al., 1997. Mol. Microbiol. 24, 93–104). The deduced proteins of the last two genes display a high sequence similarity to heavy metal or multidrug efflux pumps of the RND ( R esistance/ N odulation/cell D ivision)-MFP ( M embrane F usion P rotein) family. Reverse transcription and polymerase chain reaction (RT-PCR) analysis demonstrated that ragC is cotranscribed with rpoH 3 . Mutant strains carrying disrupted rag genes or an extented deletion of the rag locus exhibited neither an apparent growth defect nor a deficiency in the symbiotic interaction with soybean roots. The minimal inhibitory concentrations (MICs) of various metals and organic compounds were identical for the wild-type and mutant strains. Moreover, translational lacZ fusions to each of the first four genes of the rag cluster showed a very low expression under all conditions tested; hence, the substrate for the putative efflux pump has not been uncovered.