The two-component system PvgAS orchestrates virulence gene expression in response to osmolality in Pseudomonas...

Pseudomonas plecoglossicida is an important pathogen causing diseases in various fish including large yellow croaker leading to severe economic losses. Type VI secretion system (T6SS) has been established essential for its invasion and colonization in hosts, but the mechanism underlying regulation of virulence gene expression in vivo in physical conditions is still lacking. In this study, we identified that histidine kinase PvgS and response regulator PvgA consist a cognate two-component system (TCS) that has been established to regulate expression of key virulence genes including T6SS-1. Chromatin immuno-precipitation sequencing (ChIP-seq) technology, qRT-PCR and electrophoretic mobility shift assay (EMSA) revealed the specific PvgA binding Logos present in the ∼106 genes directly controlled by PvgA in P. plecoglossicida. Structural models of PvgS bound to Na⁺ and K⁺ ions and mutation analysis indicated that PvgS mediated an osmolality dependent virulence gene expression, i.e. T6SS-1 and pvgAS. PvgAS switch T6SS-1 expression by responding to different osmolality of Na⁺, K⁺, or Li⁺, but not sucrose. Moreover, we showed distinct roles of osmolality and temperature in the hierarchical regulatory mechanism to control the virulence gene expression in P. plecoglossicida, i.e. while temperature synergistically affects the T6SS-1 secretion at low osmotic environment, osmotic pressure dominates the expression of T6SS-1 at both high and low temperatures. Taken together, our study provided a new paradigm for PvgAS mediated virulence gene expression in P. plecoglossicida by responding to ion mediated osmolality variations, and facilitated the understanding of its in vivo and in vitro lifestyle switching and bacterial pathogenesis.