The LetE protein enhances expression of multiple LetA/LetS-dependent transmission traits by Legionella pneumophila

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 approximately 12-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.