FIG 8.

Model of the regulatory systems that control the expression of the L. pneumophila icm/dot genes and effector-encoding genes. The three TCSs (CpxRA, PmrAB, and LetAS), the components of the LetAS-RsmYZ-CsrA regulatory cascade, and the two Fis regulators (Fis1 and Fis3) are schematically illustrated. The environmental signals sensed by CpxA and PmrB are currently not known, and the phosphorylation of these components is expected to be activated by transfer of the phosphate group to their cognate response regulators CpxR and PmrA, respectively, which then directly activate or repress the transcription of their target effector-encoding genes. During stationary phase, the LetAS TCS activates the expression of the small RNAs RsmY and RsmZ that thus sequester CsrA from its target mRNAs and relieve the CsrA posttranscriptional repression. The csrA gene was shown to be under the regulation of the PmrA transcriptional regulator. The numbers of effector-encoding genes which were shown to be regulated by each of the regulatory systems are indicated in parentheses. Solid lines and dashed lines indicate direct and indirect regulation, respectively. Arrows and T-shaped symbols indicate activation and repression, respectively.