The Legionella pneumophila global regulatory protein LetA affects DotA and Mip

Several genes have been identified in Legionella pneumophila which are necessary for its virulence properties. These genes include the dot/icm type IV secretion system (T4SS), mip and letA. Genes of the dot/icm system, in particular dotA, have been found to be essential for intracellular growth. The macrophage infectivity protein (Mip) is also necessary for full virulence of the bacteria. Although these genes are well characterized, the regulation of such virulence factors is not. The LetA transcriptional activator interacts with the global regulator CsrA in controlling the switch from the replicative, non-infectious to the transmissive, highly infectious form of L. pneumophila. Regulation by LetA of the dot/icm genes has also been previously postulated. Here we show that the letA mutation exerts effects not only on DotA but on a substrate of the secretion system, RalF as well. LetA was found to be necessary for full transcriptional expression of the dotA and ralF genes. Although at the transcriptional level dotA was reduced, this did not result in a decrease of DotA protein in whole cell lysates. The letA mutation, however, does result in decreased amounts of the DotA protein found in the membrane and increased amounts in the culture supernatant. Additionally, the letA mutation dramatically decreased the secretion of Mip. This work demonstrates the participation of the global regulatory protein LetA in the regulation of an essential part of the dot/icm T4SS. Also shown is the presence of secreted Mip and a decrease in this secretion in the letA(−) strain. Exactly how LetA is regulating these virulence factors remains to be elucidated but it obviously occurs at both transcriptional and post-transcriptional levels.