The transcriptional regulator PsrA links to the stringent response network governing virulence in Legionella pneumophila

Abstract

Legionella pneumophila, a bacterium widely found in freshwater environments including anthropogenic infrastructure, is an intracellular parasite of co-habitating protozoa, but also an opportunistic human pathogen and the causative agent of a severe pneumonia termed Legionnaires’ disease. Bacterial uptake mechanisms are similar in both protozoa and human macrophages, where L. pneumophila enters the cell, then subsequently establishes a replicative niche while using the host cell as a source of nutrients. The fate of the intracellular bacteria is host dependent. In protozoa, nutrient depletion triggers cessation of replication and the transition to transmissive phase, including bacterial differentiation into resilient and infectious cyst forms released upon host cell lysis. In macrophages, bacterial differentiation is interrupted by apoptosis which results in the release of immature bacterial forms. The regulatory mechanisms governing the transition to transmissive phase are shared in vitro and in vivo. They are organized into a complex regulatory network centred on the stringent response that, along with stationary-phase sigma factor RpoS, monitors the nutritional state of the environment. This study characterizes L. pneumophila PsrA (LpPsrA), an orthologue of PsrA, an activator of rpoS expression in Pseudomonas aeruginosa. Despite its homology, LpPsrA diverges from PsrA in function and recognition of binding site sequences, resulting in differing regulatory gene targets. Notably, LpPsrA does not regulate rpoS expression in L. pneumophila. LpPsrA showed a clear role in controlling the temporal progression of intracellular development in Acanthamoeba castellanii protozoa. A ΔpsrA deletion mutant strain showed reduced growth with bacteria prematurely differentiated into cyst forms. Conversely, elevated levels of cellular LpPsrA resulted in unusual intracellular formation of enlarged vacuoles where cyst differentiation was inhibited. Transcriptomic analyses indicate a diverse LpPsrA regulon laden with genes associated with motility and virulence effectors, providing insight on the underlying mechanisms including linkage to the stringent response-based network. Supporting this, a fortuitous dysfunctional mutation in stringent response cofactor DksA revealed a reciprocal co-regulatory relationship between PsrA and DksA in terms of fatty acid response, cellular morphology, and pigmentation. Taken together, this highlights a broader integration of LpPsrA as a crucial contributor to the stringent response driven regulatory network in L. pneumophilia.