Control of Flagellar Gene Regulation in Legionella pneumophila and Its Relation to Growth Phase

Abstract

Bacterial flagella are highly complex molecular machines. They are surface organelles assembled from over 40 different protein components that mediate bacterial motility. To ensure maximal efficiency and accuracy during flagellar biogenesis, bacteria use hierarchical regulatory networks involving transcriptional and posttranscriptional mechanisms to control the ordered expression of the individual components of the flagellar organelle. Although significant differences exist between the regulatory mechanisms used by different bacteria, a salient feature in all cases is that the flagellar genes can be classified based upon their temporal gene expression and on their dependence on various nested transcriptional regulators (for a recent review, see reference 33).The bacterial pathogen Legionella pneumophila lives in natural and manmade water systems and replicates intracellularly within aquatic protozoa (41). When inhaled by humans, L. pneumophila is able to survive and replicate within alveolar macrophages (28). After entry into host cells, L. pneumophila inhibits phagolysosomal fusion (26, 27) and establishes a specialized Legionella-containing vacuole (LCV) surrounded by endoplasmic reticulum in which L. pneumophila represses transmissive traits and starts to replicate (15,37,43). During the bacterial late replicative phase, the LCV merges with lysosomes (44). Finally, induced by a nutrient decline the bacteria enter the transmissive phase, which is reflected by a major shift in gene expression (2,8,14,19,37,51). In the transmissive phase, L. pneumophila expresses many virulence-associated traits promoting the release of the bacteria and infection of a new host (2,3,23,36,42,45,46,51). One striking feature of transmissive L. pneumophila is the expression of a single monopolar flagellum composed of the flagellin subunit FlaA. The flagellum mediates invasivness of L. pneumophila for human macrophage-like cell lines and cytotoxicity to macrophages (13,20). Furthermore, it was shown that flagellin sensed by nonpermissive mouse macrophages mediates cell death by activating the cytosolic Naip5 (Birc1e) receptor (35,40). Expression of the flagellum is dependent on the regulatory circuit controlling phase transition (for a review, see reference 1) and different environmental factors (21,22).Several studies have been undertaken to understand the regulatory mechanisms governing this life cycle switch, including the regulation of flagellar gene expression. The two-component system LetA/LetS, a system homologous to BarA/UvrY of Escherichia coli and RsmA/RsmS of Pseudomonas aeruginosa, was shown to have an important role in the regulation of the life cycle switch and in flagellar gene expression (17,20,32,36,42). It is suggested that LetA/LetS responds to the alarmone molecule (p)ppGpp, synthesized by RelA and SpoT (8,

FliA expression analysis and influence of the regulatory proteins RpoN, FleQ and FliA on virulence and in vivo fitness in Legionella pneumophila

“…Moreover, FleQ and RpoN seemed not to be reduced in their infection cycle within Acanthamoeba castellanii (Albert-Weissenberger et al 2010;Jacobi et al 2004). This is surprising since not only do FleQ and RpoN influence the expression of flagellar genes, but they are also involved in the expression of various known virulence genes of L. pneumophila Paris (Albert-Weissenberger et al 2010).…”

“…Beneath these global regulators influencing flagellin expression, the flagellar regulon of L. pneumophila is organized as a regulatory cascade exhibiting four different classes of genes (Albert-Weissenberger et al 2010;Heuner and Albert-Weissenberger 2008). The master regulator FleQ (sigma factor 54 activator protein, class I gene) and the alternative 54 factor (RpoN) together enhance the expression of the flagellar class II genes (predominantly basal body genes).…”

“…The alternative 28 factor FliA (class III gene) directly controls the expression of the flagellar class IV genes (e.g. flaA) (Albert-Weissenberger et al 2010;Brüggemann et al 2006;Jacobi et al 2004;Heuner and Albert-Weissenberger 2008). It is known that L. pneumophila rpoN, fleQ, fliA and flaA mutant strains are non-flagellated and non-motile (Heuner et al 1995;Heuner et al 2002;Heuner et al 1997;Jacobi et al 2004;Molofsky et al 2005).…”

“…Conflicting data suggested different regulation mechanisms in strains Corby and Paris for fliA expression. It seems as if fliA belongs to the class III genes in strain Paris, but is not directly linked to the flagellar regulon in strain Corby (Albert-Weissenberger et al 2010;Jacobi et al 2004). Both strains are virulent L. pneumophila Sg1 isolates with only small differences within their genomes (Cazalet et al 2004;Jepras et al 1985;Glöckner et al 2008).…”

“…FliA is required for the in vivo fitness (Albert-Weissenberger et al 2010;Dietrich et al 2001;Hammer et al 2002;Heuner et al 2002;Jacobi et al 2004;Molofsky et al 2005). Furthermore it is thought that motility but not flagellin promotes contact with host cells and increases infectivity (Molofsky et al 2005).…”