Dual role of CsrA in regulating the hemolytic activity of O157:H7.

Post-transcriptional global carbon storage regulator A (CsrA) is a sequence-specific RNA-binding protein involved in the regulation of multiple bacterial processes. Hemolysin is an important virulence factor in the enterohemorrhagic O157:H7 (EHEC). Here, we show that CsrA plays a dual role in the regulation of hemolysis in EHEC. CsrA significantly represses plasmid-borne enterohemolysin (EhxA)-mediated hemolysis and activates chromosome-borne hemolysin E (HlyE)-mediated hemolysis through different mechanisms. RNA structure prediction revealed a well-matched stem-loop structure with two potential CsrA binding sites located on the 5' untranslated region (UTR) of , which encodes a translocator required for EhxA secretion. CsrA inhibits EhxA secretion by directly binding to the RNA leader sequence of to repress its expression in two different ways: CsrA either binds to the Shine-Dalgarno sequence of to block ribosome access or to transcript to promote its mRNA decay. The predicted CsrA-binding site 1 of is essential for its regulation. There is a single potential CsrA-binding site at the 5'-end of the transcript, and its mutation completely abolishes CsrA-dependent activation. CsrA can also stabilize mRNA by directly binding to its 5' UTR. Overall, our results indicate that CsrA acts as a hemolysis modulator to regulate pathogenicity under certain conditions.