Transcriptional Regulator LsrB of Sinorhizobium meliloti Positively Regulates the Expression of Genes Involved in Lipopolysaccharide Biosynthesis

Rhizobia induce nitrogen-fixing nodules on host legumes, which is important in agriculture and ecology. Lipopolysaccharide (LPS) produced by rhizobia is required for infection or bacteroid survival in host cells. Genes required for LPS biosynthesis have been identified in several Rhizobium species. However, the regulation of their expression is not well understood. Here, Sinorhizobium meliloti LsrB, a member of the LysR family of transcriptional regulators, was found to be involved in LPS biosynthesis by positively regulating the expression of the lrp3-lpsCDE operon. An lsrB in-frame deletion mutant displayed growth deficiency, sensitivity to the detergent sodium dodecyl sulfate, and acidic pH compared to the parent strain. This mutant produced slightly less LPS due to lower expression of the lrp3 operon. Analysis of the transcriptional start sites of the lrp3 and lpsCDE gene suggested that they constitute one operon. The expression of lsrB was positively autoregulated. The promoter region of lrp3 was specifically precipitated by anti-LsrB antibodies in vivo. The promoter DNA fragment containing TN11A motifs was bound by the purified LsrB protein in vitro. These new findings suggest that S. meliloti LsrB is associated with LPS biosynthesis, which is required for symbiotic nitrogen fixation on some ecotypes of alfalfa plants.