Regulation of RpoS by a novel small RNA: the characterization of RprA

Translational regulation of the stationary phase sigma factor RpoS is media ted by the formation of a double-stranded RNA stem-loop structure in the up stream region of the rpoS messenger RNA, occluding the translation initiati on site. The interaction of the rpoS mRNA with a small RNA, DsrA, disrupts the double-strand pairing and allows high levels of translation initiation. We screened a multicopy library of Escherichia coli DNA fragments for nove l activators of RpoS translation when DsrA is absent. Clones carrying rprA (RpoS regulator RNA) increased the translation of RpoS. The rprA gene encod es a 106 nucleotide regulatory RNA. As with DsrA, RprA is predicted to form three stem-loops and is highly conserved in Salmonella and Klebsiella spec ies. Thus, at least two small RNAs, DsrA and RprA, participate in the posit ive regulation of RpoS translation. Unlike DsrA, RprA does not have an exte nsive region of complementarity to the RpoS leader, leaving its mechanism o f action unclear. RprA is non-essential. Mutations in the gene interfere wi th the induction of RpoS after osmotic shock when DsrA is absent, demonstra ting a physiological role for RprA. The existence of two very different sma ll RNA regulators of RpoS translation suggests that such additional regulat ory RNAs are likely to exist, both for regulation of RpoS and for regulatio n of other important cellular components.