Contribution of a Novel TetR/AcrR Family Transcriptional Regulator RalT of Ralstonia pseudosolanacearum Strain OE1-1 to the Fine-Tuning of Its Virulence.

During the quorum-sensing (QS)-active state, the Gram-negative phytopathogenic Ralstonia pseudosolanacearum strain OE1-1 activates the transcriptional regulator PhcA, regulating the QS-dependent genes including ralfuranone production-related genes such as ralA and major exopolysaccharide EPS I production-related genes such as xpsR encoding the transcriptional regulator XpsR, which are responsible for OE1-1 virulence. Ralfuranone affects the regulation of more than 80% (QS/Ral-dependent genes) of QS-dependent genes, indicating the ralfuranone-mediated feedback regulation of QS. To elucidate the mechanisms underlying the regulation of QS/Ral-dependent genes, we analyzed the transcriptomes of phcA-deletion (ΔphcA) and ralfuranone-deficient (ΔralA) mutants, as well as strain OE1-1 by RNA-sequencing. We found a novel TetR/AcrR family transcriptional regulator (RalT)-encoding gene (ralT); the expression level of ralT reduced significantly in ΔralA but not ΔphcA relative to expression level in strain OE1-1, and RalT negatively regulated ralT. A transcriptome analysis of the ralT-deletion mutant (ΔralT) showed that the ralT-deletion reduced the expression levels of 89.4% of positively QS/Ral-dependent genes including ralA and xpsR, while enhanced 44.6% of negatively QS/Ral-dependent genes. The transcript levels of these genes were positively correlated between ΔralT and ΔphcA or ΔralA, suggesting contribution of RalT to the regulation of some QS/Ral-dependent genes. However, the ralT-deletion enhanced EPS I production, suggesting that RalT represses the XpsR-independent factor(s), which is regulated PhcA and contributes to the EPS I production. Furthermore, ΔralT exhibited enhanced virulence, compared with strain OE1-1. Collectively, results of the present genetic study suggest that RalT contributes to the exquisite fine-tuning of OE1-1 virulence.