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

ABSTRACT 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 (Δ phcA ) and ralfuranone‐deficient (Δ ralA ) 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 Δ ralA but not Δ phcA relative to expression level in strain OE1‐1, and RalT negatively regulated ralT . A transcriptome analysis of the ralT‐ deletion mutant (Δ ralT ) 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 Δ ralT and Δ phcA or Δ ralA , 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, Δ ralT 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.