The LetE Protein Enhances Expression of Multiple LetA/LetS-Dependent Transmission Traits by Legionella pneumophila

Abstract

Legionella pneumophila colonizes freshwater amoebae and can also replicate within alveolar macrophages. When their nutrient supply is exhausted, replicating bacteria become cytotoxic, motile, and infectious, which is thought to promote transmission to a new amoeba. The differentiation of L. pneumophila is coordinated by the sigma factors RpoS and FliA and the two-component regulator LetA/LetS and is enhanced by the letE locus. Here we demonstrate that letE promotes motility by increasing expression of the flagellin gene flaA but has little impact on the transcription of fliA, the flagellar sigma factor gene. In addition to promoting motility, letE induces the characteristic shape, pigment, and heat resistance of stationary-phase L. pneumophila. To gain insight into how letE promotes the expression of the transmission phenotype, we designed molecular genetic experiments to discriminate between the following three models: letE mutations are polar on milX; letE encodes a small novel protein; or, by analogy to csrB, letE encodes a regulatory RNA that sequesters CsrA to relieve repression. We report that letE encodes an activator protein, as it does not complement an Escherichia coli csrB mutant, it directs the synthesis of an ϳ12-kDa polypeptide, and a letE nonsense mutation eliminates function. A monocistronic letE RNA is abundant during the exponential phase, and its decay during the stationary phase requires RpoS and LetA/LetS. We also discuss how the LetE protein may interact with LetA/LetS and CsrA to enhance L. pneumophila differentiation to a transmissible form.

Starvation- and antibiotics-induced formation of persister cells in Pseudomonas aeruginosa

“…On the other hand, numbers of persisters were the highest in the wild-type strain (0.00156%), ∆relA (~0.0003%), ∆rpoS (~0.00017%) and ∆relAspoT (0.000015%), when mid-exponential phase cells untreated with SHX were exposed to ciprofloxacin at ten-fold the MIC (data not shown). It has been described that rpoS transcripts were more abundant during in the exponential phase than in the stationary phase of Legionella pneumophila 39 . However, high stress or stationary phase upregulates the expression of relA and spoT genes, whereby ppGpp levels accumulate 40 .…”

ppGpp Conjures Bacterial Virulence

“…RpoS is essential for the intracellular replication of L. pneumophila in Acanthamoeba castellani and is partially required for growth in macrophages (12,91). Although RpoS translation efficiency and protein levels in L. pneumophila have not been monitored, the rpoS transcript is more abundant in the exponential phase than in the stationary phase (13). As with E. coli, the transcription of rpoS in L. pneumophila is sensitive to ppGpp pools, since transcript levels increased following the artificial induction of the alarmone (30).…”

Regulation of Bacterial Virulence by Csr (Rsm) Systems

“…In many bacterial species, the regulatory activities of CsrA/ RsmA and RpoS appear to oppose one another (3), yet in L. pneumophila, the LetS/LetA TCS destabilizes the rpoS transcript in stationary phase (228). Furthermore, while RpoS in other bacterial species promotes stationary-phase resistance to various environmental stresses (229), RpoS in L. pneumophila does not appear to perform this function (197,230).…”