Genetic Evidence that Legionella pneumophila RpoS Modulates Expression of the Transmission Phenotype in Both the Exponential Phase and the Stationary Phase

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Abstract
The opportunistic pathogen Legionella pneumophila alternates between two states: replication within phagocytes and transmission between host amoebae or macrophages. In broth cultures that model this life cycle, during the replication period, CsrA inhibits expression of transmission traits. When nutrients become limiting, the alarmone (p)ppGpp accumulates and the sigma factors RpoS and FliA and the positive activators LetA/S and LetE promote differentiation to the transmissible form. Here we show that when cells enter the postexponential growth phase, RpoS increases expression of the transmission genes fliA, flaA, and mip, factors L. pneumophila needs to establish a new replication niche. In contrast, in exponential (E)-phase cells whose (p)ppGpp levels are low, rpoS inhibits expression of transmission traits, on the basis of three separate observations. First, rpoS RNA levels peak in the E phase, suggestive of a role for RpoS during replication. Second, in multiple copies, rpoS decreases the amounts of csrA, letE, fliA, and flaA transcripts and inhibits the transmission traits of motility, infectivity, and cytotoxicity. Third, rpoS blocks expression of cytotoxicity and motility by E-phase bacteria that have been induced to express the LetA activator ectopically. The data are discussed in the context of a model in which the alarmone (p)ppGpp enables RpoS to outcompete other sigma factors for binding to RNA polymerase to promote transcription of transmission genes, while LetA/S acts in parallel to relieve CsrA posttranscriptional repression of the transmission regulon. By coupling transcriptional and posttranscriptional control pathways, intracellular L. pneumophila could respond to stress by rapidly differentiating to a transmissible form.