The LetA-RsmYZ-CsrA regulatory cascade, together with RpoS and PmrA, post-transcriptionally regulates stationary phase activation of Legionella pneumophila Icm/Dot effectors.

Legionella pneumophila utilize the Icm/Dot type-IV secretion system to translocate effector proteins into host cells. Some of these effectors were shown before to be regulated at the transcriptional level by the PmrAB and CpxRA two-component systems. In addition, the stationary phase-related regulators LetA and CsrA, which are both members of the same post-transcriptional regulatory cascade, were shown to be involved in L. pneumophila virulence. In this report, we identified two small non-coding RNAs which are part of the LetA-CsrA regulatory cascade and three effector-encoding genes which are directly controlled by this regulatory system. We found that the small non-coding RNAs RsmY and RsmZ, were upregulated by LetA at stationary phase, and relieve the repression of CsrA from its target genes. The three effector-encoding genes were found to be post-transcriptionally upregulated at stationary phase and to contain CsrA regulatory elements that were found to be essential for their stationary phase activation. In addition, rsmY and rsmZ were found to be regulated by the RpoS sigma-factor and the csrA encoding gene was found to be regulated by PmrA. Our results demonstrate that L. pneumophila effectors are regulated at both the transcriptional and the post-transcriptional levels by a complicated regulatory network.