Transcription Factors DksA and PsrA are synergistic contributors to L. pneumophila virulence in Acanthamoeba castellanii protozoa

Abstract

The environmental bacterium Legionella pneumophila, an intracellular parasite of free-living freshwater protozoa as well as an opportunistic human pathogen, has a biphasic lifestyle. The switch from the vegetative replicative form to the environmentally resilient transmissive phase form is governed by a complex stringent response-based regulatory network that includes RNA polymerase co-factor DksA. Here we report that, through a dysfunctional DksA mutation (DksA1), a synergistic interplay was discovered between DksA and transcription regulator PsrA using the Acanthamoeba castellanii protozoan infection model. Surprisingly, in trans expression of PsrA partially rescued the growth defect of a dksA1 strain. While in trans expression of DksA expectantly could fully rescued the growth defect of the dksA1 strain, it could also surprisingly rescue the growth defect of a {Delta}psrA strain. Conversely, the severe intracellular growth defect of a {Delta}dksA strain could be rescued by in trans expression of DksA and DksA1, but not PsrA. In vitro phenotypic assays show that either DksA or DksA1 were required for extended culturability of bacterial cells, but normal cell morphology and pigmentation required DksA only. Comparative structural modeling predicts that the DksA1 mutation affects coordination of Mg2+ into the active site of RNAP compromising transcription efficiency. Taken together, we propose that PsrA transcriptionally assists DksA in the expression of select transmissive phase traits. Additionally, in vitro evidence suggests that the long chain fatty chain metabolic response is mediated by PsrA together with DksA inferring a novel regulatory link to the stringent response pathway.