Legionella pneumophila RpoS (LpRpoS), an alternative sigma factor of RNA polymerase (RNAP), is essential for virulence and stress resistance. To investigate the mechanism of RpoS in the intracellular pathogen L. pneumophila, we determined the high-resolution crystal structure of the LpRpoS 95–195 containing a partial region 1.2 and region 2. The structure of LpRpoS 95–195 reveals that the conserved residues are critical for promoter melting, DNA and core RNAP binding. The differences in regulatory factor binding site between Escherichia coli RpoS and LpRpoS suggest that LpRpoS may employ a distinct mechanism to recruit alternative regulatory factors controlling transcription initiation.
