The sensor histidine kinase PhcS participates in the regulation of quorum sensing-dependent virulence genes in Ralstonia pseudosolanacearum strain OE1-1.

Ralstonia pseudosolanacearum strain OE1-1 secretes methyl 3-hydroxymyristate (3-OH MAME) as a quorum-sensing (QS) signal. Strain OE1-1 senses the chemical by the sensor histidine kinase PhcS, leading to the activation of the LysR family transcriptional regulator PhcA. The activated PhcA controls the expression of QS-dependent genes responsible for QS-regulated phenotypes including virulence. The autophosphorylation of the histidine at amino acid position 230 (H230-PhcS) in PhcS following the 3-OH MAME sensing is required for the PhcA activation. The alternative sensor histidine kinase PhcK is involved in the regulation of phcA, which is independent of 3-OH MAME sensing. Furthermore, the H230Q-PhcS substitution of H230-PhcS with glutamine significantly decreases phcA expression. However, how PhcK and PhcS regulate phcA expression remains unclear. To elucidate the mechanisms of the phcA regulation, we generated a phcK mutant with the H205Q-PhcK substitution of autophosphorylated histidine at amino acid position 205 of PhcK with glutamine. A transcriptome analysis using quantitative real-time polymerase chain reaction assay and RNA sequencing showed that the H230Q-PhcS substitution, but not the H205Q-PhcK substitution, significantly decreased the expression level of phcA. The H230Q-PhcS substitution led to significant changes in the expression levels of QS-dependent genes and a loss of virulence, similar to phcA or phcK deletion. It is thus thought that PhcS participates in not only the 3-OH MAME sensing-independently PhcK-mediated regulation of phcA but also the PhcA activation following 3-OH MAME sensing. Both functions of PhcS are significantly influenced by the autophosphorylation of H230-PhcS.

Importance: The soil-borne Ralstonia solanacearum species complex (RSSC) infects more than 300 plant species in over 50 families, including solanaceous plants, causing the devastating wilt disease that substantially decreases agricultural production worldwide. The cell density-dependent gene regulation system, QS, is required for RSSC virulence and involves two signaling pathways for the induction and activation of PhcA, which is the master transcriptional regulator in QS. In the present study, we describe the contribution of sensor histidine kinase PhcS to the PhcA induction, along with the alternative sensor kinase PhcK, independently of the sensing of QS signal methyl 3-hydroxymyristate in a phylotype I strain of RSSC, R. pseudosolanacearum strain OE1-1. This study further expands our knowledge of multiple networks, suggesting that several PhcS-mediated two-component systems are likely necessary for RSSC QS and virulence.