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Controlling iron homeostasis is crucial for all aerobically grown living cells that are exposed to oxidative damage by reactive oxygen species (ROS), as free iron increases the production of ROS. Methionine sulfoxide reductases (Msr) are key enzymes in repairing ROS-mediated damage to proteins, as they reduce oxidized methionine (MetSO) residues to methionine.  E. coli  synthesizes two Msr, A and B, which exhibit substrate diastereospecificity. The bacterial iron-responsive small RNA (sRNA) RyhB controls iron metabolism by modulating intracellular iron usage. We show in this paper that RyhB is a direct regulator of the  msrB  gene that encodes the MsrB enzyme. RyhB down-regulates  msrB  transcripts along with Hfq and RNaseE proteins since mutations in the  ryhB ,  fur ,  hfq , or RNaseE-encoded genes resulted in iron-insensitive expression of  msrB . Our results show that RyhB binds to two sequences within the short 5′UTR of  msrB  mRNA as identified by reverse transcriptase and RNase and lead (II) protection assays. Toeprinting analysis shows that RyhB pairing to  msrB  mRNA prevents efficient ribosome binding and thereby inhibits translation initiation.  In vivo  site directed-mutagenesis experiments in the  msrB  5′UTR region indicate that both RyhB-pairing sites are required to decrease  msrB  expression. Thus, this study suggests a novel mechanism of translational regulation where a same sRNA can basepair to two different locations within the same mRNA species. In contrast, expression of  msrA  is not influenced by changes in iron levels.

The sRNA RyhB Regulates the Synthesis of the Escherichia coli Methionine Sulfoxide Reductase MsrB but Not MsrA