Conjugal transfer of the S inorhizobium meliloti 1021 symbiotic plasmid is governed through the concerted action of one‐ and two‐component signal transduction regulators

Summary Conjugal transfer of S inorhizobium meliloti and R hizobium 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 SM a0974 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 ( SM a0955) 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 ( SM a0961) 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.