Functional Analysis of the Alternative Sigma-28 Factor FliA and Its Anti-Sigma Factor FlgM of the Nonflagellated Legionella Species L. oakridgensis.

causes Legionnaires' disease but is known to be less virulent than is one of the species that is nonflagellated. The genes of the flagellar regulon are absent, except those encoding the alternative sigma-28 factor (FliA) and its anti-sigma-28 factor (FlgM). Similar to , and , located in the same phylogenetic clade, have no flagellar regulon, although both are positive for and Here, we investigated the role and function of both genes to better understand the role of FliA, the positive regulator of flagellin expression, in nonflagellated strains. We demonstrated that the FliA gene of encodes a functional sigma-28 factor that enables the transcription start from the sigma-28-dependent promoter site. The investigations have shown that FliA is necessary for full fitness of Interestingly, expression of FliA-dependent genes depends on the growth phase and temperature, as already shown for strains that are flagellated. In addition, we demonstrated that FlgM is a negative regulator of FliA-dependent gene expression. FlgM seems to be degraded in a growth-phase- and temperature-dependent manner, instead of being exported into the medium as reported for most bacteria. The degradation of FlgM leads to an increase of FliA activity. A less virulent species, , causes Legionnaires' disease and is known to not have flagella, even though has the regulator of flagellin expression (FliA). This protein has been shown to be involved in the expression of virulence factors. Thus, the strain was chosen for use in this investigation to search for FliA target genes and to identify putative virulence factors of One of the five major target genes of FliA identified here encodes the anti-FliA sigma factor FlgM. Interestingly, in contrast to most homologs in other bacteria, FlgM in seems not to be transported from the cell so that FliA gets activated. In , FlgM seems to be degraded by protease activities.