The symbiosis regulator CbrA modulates a complex regulatory network affecting the flagellar apparatus and cell envelope proteins.

Sinorhizobium meliloti participates in a nitrogen-fixing symbiosis with legume plant host species of the genera Medicago, Melilotus, and Trigonella. We recently identified an S. meliloti two-component sensory histidine kinase, CbrA, which is absolutely required to establish a successful symbiosis with Medicago sativa (K. E. Gibson, G. R. Campbell, J. Lloret, and G. C. Walker, J. Bacteriol. 188:4508-4521, 2006). In addition to having a symbiotic defect, the cbrA::Tn5 mutant also has free-living phenotypes that suggest a cell envelope perturbation. Because the bases for these phenotypes are not well understood, we undertook an identification of CbrA-regulated genes. We performed a microarray analysis and compared the transcriptome of the cbrA::Tn5 mutant to that of the wild type. Our global analysis of gene expression identified 162 genes that are differentially expressed in the cbrA::Tn5 mutant, including those encoding proteins involved in motility and chemotaxis, metabolism, and cell envelope function. With regard to those genes with a known role in symbiosis, we observed increased expression of nine genes with overlapping functions in bacterial invasion of its host, which suggests that the mutant could be competent for invasion. Since these CbrA-repressed genes are vital to the invasion process, it appears that down-regulation of CbrA activity is important at this stage of nodule development. In contrast, our previous work showed that CbrA is required for bacteria to establish themselves within the host as nitrogen-fixing symbionts. Therefore, we propose a model in which CbrA functions as a developmental switch during symbiosis.