A complex network of small non-coding RNAs regulate motility in Escherichia coli.

Small Hfq-dependent non-coding regulatory RNAs (sRNAs) that alter mRNA stability and expression by pairing with target mRNAs have increasingly been shown to be important in influencing the behaviour of bacteria. In Escherichia coli, flhD and flhC, which encode the master regulator of flagellar synthesis, are co-transcribed from a promoter that is regulated by multiple transcription factors that respond to different environmental cues. Here, we show that the 5' untranslated region (5' UTR) of the flhDC mRNA also serves as a hub to integrate additional environmental cues into the decision to make flagella. Four sRNAs, ArcZ, OmrA, OmrB and OxyS, negatively regulated and one sRNA, McaS, positively regulated motility and flhDC expression by base-pairing with the 5' UTR of this mRNA. Another sRNA, MicA, positively regulated motility independent of regulation of flhDC. Furthermore, we demonstrate that the regulation of motility by the ArcB/A two component system is in part due to its regulation of ArcZ. flhDC is the first mRNA that has been shown to be both positively and negatively regulated by direct pairing to sRNAs. Moreover, both positive regulation by McaS and negative regulation by ArcZ require the same binding site in the flhDC mRNA.