Conjugal transfer of the Sinorhizobium meliloti 1021 symbiotic plasmid is governed through the concerted action of one- and two-component signal transduction regulators.

Conjugal transfer of Sinorhizobium meliloti and Rhizobium etli symbiotic plasmids are repressed by the transcriptional regulator RctA. Here we report on new key players in the signal transduction cascade towards S. meliloti pSym conjugation. We have identified S. meliloti pSymA gene SMa0974 as an orthologue of the R. etli rctB gene which is required to antagonize repression by RctA. In S. meliloti two additional genes, rctR and rctC participate in control of rctB expression. rctR (SMa0955) encodes a protein of the GntR family of transcriptional regulators involved in repression of rctB. A rctR mutant promotes pSymA conjugal transfer and displays increased transcription of tra, virB and rctB genes even in presence of wild-type rctA gene. Among genes repressed by RctR, rctC (SMa0961) encodes a response regulator required to activate rctB transcription and therefore for derepression of plasmid conjugative functions. We conclude that in both R. etli and S. meliloti pSym conjugal transfer is derepressed via rctB, however the regulatory cascades to achieve activation of rctB are probably different. Upstream of rctB, the S. meliloti pSym conjugal transfer is regulated through the concerted action of genes representing one- (rctR) and two-component (rctC) signal transduction systems in response to yet unidentified signals.