A predatory soil bacterium reprograms a quorum sensing signal system to regulate antifungal weapon production in a cyclic-di-GMP-independent manner

Abstract

Soil bacteria often provide multiple weapons for eukaryotes or prokaryotes to use against predators. Diffusible signal factors (DSFs) represent a unique group of quorum sensing (QS) chemicals that modulate interspecies competition in bacteria that do not produce antibiotic-like molecules. However, the molecular mechanism by which DSF-mediated QS systems regulate weapons production for interspecies competition remains largely unknown in soil biocontrol bacteria. In this study, we found that the necessary QS system component protein RpfG from Lysobacter, in addition to being a cyclic dimeric GMP (c-di-GMP) phosphodiesterase (PDE), regulates the biosynthesis of an antifungal weapon (heat-stable antifungal factor, HSAF), which does not appear to depend on the enzymatic activity. Interestingly, we showed for the first time that RpfG interacts with three hybrid two-component system (HyTCS) proteins, HtsH1, HtsH2, and HtsH3, to regulate HSAF production in Lysobacter. In vitro studies showed that each of these proteins interacted with RpfG, which reduced the PDE activity of RpfG. Finally, we showed that the cytoplasmic proportions of these proteins depended on their phosphorylation activity and binding to the promoter controlling the genes implicated in HSAF synthesis. These findings reveal a new mechanism of DSF signalling in antifungal weapon production in soil bacteria.