Hfq is the only known chaperone for trans-acting small RNAs (sRNAs) in Gram-negative bacteria. However, Attaiech et al. show that RocC and RocR are a chaperone and trans-acting sRNA, respectively, that together mediate post-transcriptional silencing of competence in the human pathogen Legionella pneumophila. Silencing by RocC–RocR seems to occur by a mechanism similar to that commonly mediated by Hfq–sRNA: RocC enables duplex formation between RocR and the 5′ UTR of target mRNAs, thereby masking the ribosome binding site. Interestingly, RocC–RocR provides the first example of post-transcriptional regulation of competence, which is otherwise only known to be regulated by transcriptional activators. Although RocR and RocC specifically silenced genes that are involved in competence, the ProQ/FinO domain, which was responsible for sRNA binding by RocC, is widespread in Proteobacteria and, the authors suggest, might define a class of RNA chaperones that mediate sRNA silencing.
Attaiech, L. et al. Silencing of natural transformation by an RNA chaperone and a multitarget small RNA. Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1601626113 (2016)
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Attar, N. A new class of Hfq-like sRNA chaperones?. Nat Rev Microbiol 14, 546 (2016). https://doi.org/10.1038/nrmicro.2016.118