Control of iron acquisition by multiple small RNAs unravels a new role for transcriptional terminator loops in gene regulation

Small RNAs (sRNAs) controlling gene expression by imperfect base-pairing with mRNA(s) are widespread in bacteria and regulate multiple genes, including genes involved in iron homeostasis, through a wide variety of mechanisms. We previously showed that OmrA and OmrB sRNAs repress the synthesis of the Escherichia coli FepA receptor for iron-enterobactin complexes. We now report that five additional sRNAs, namely RprA, RybB, ArrS, RseX and SdsR, that respond to different environmental cues, also repress fepA, independently of one another. While RprA follows the canonical mechanism of pairing with the translation initiation region, repression by ArrS or RseX requires a secondary structure far upstream within the long fepA 5UTR. We also demonstrate a dual action of SdsR, whose 5 end pairs with the fepA translation initiation region while its 3 end behaves like ArrS or RseX. Strikingly, mutation analysis shows a key role for the loops of these sRNAs intrinsic terminators in the regulation. Regulation furthermore depends on both the Hfq chaperone and the RNase E endonuclease. Overall, our data strongly suggest that FepA levels must be tightly controlled under a variety of conditions, and highlight the diversity of mechanisms that underly the regulation of gene expression by sRNAs in bacteria.