Summary: Small RNAs (sRNAs), particularly those that act by limited base pairing with mRNAs, are part of most regulatory networks in bacteria. In many cases, the base‐pairing interaction is facilitated by the RNA chaperone Hfq. However, not all bacteria encode Hfq and some base‐pairing sRNAs do not require Hfq raising the possibility of other RNA chaperones. Candidates are proteins with homology to FinO, a factor that promotes base pairing between the FinP antisense sRNA and the traJ mRNA to control F plasmid transfer. Recent papers have shown that the Salmonella enterica FinO‐domain protein ProQ binds a large suite of sRNAs, including the RaiZ sRNA, which represses translation of the hupA mRNA, and the Legionella pneumophila protein RocC binds the RocR sRNA, which blocks expression of competence genes. Here we discuss what is known about FinO‐domain structures, including the recently solved Escherichia coli ProQ structure, as well as the RNA binding properties of this family of proteins and evidence they act as chaperones. We compare these properties with those of Hfq. We further summarize what is known about the physiological roles of FinO‐domain proteins and enumerate outstanding questions whose answers will establish whether they constitute a second major class of RNA chaperones.

The conserved A32-U38 pair in the anticodon loop of tRNAAlaGGC is now shown to be important for accurate decoding. Pre–steady state kinetic analyses of mutants in A32-U38 show that they can efficiently decode near-cognate codons, with a mismatch in any of the three positions, pointing to the role of such conserved sequence elements in suppressing misreading during translation. Mutating the rare A32-U38 nucleotide pair at the top of the anticodon loop of Escherichia coli tRNAAlaGGC to a more common U32-A38 pair results in a tRNA that performs almost normally on cognate codons but is unusually efficient in reading near-cognate codons. Pre–steady state kinetic measurements on E. coli ribosomes show that, unlike the wild-type tRNAAlaGGC, the misreading mutant tRNAAlaGGC shows rapid GTP hydrolysis and no detectable proofreading on near-cognate codons. Similarly, tRNAAlaGGC mutated to contain C32-G38, a pair that is found in some bacterial tRNAAlaGGC sequences, was able to decode only the cognate codons, whereas tRNAAlaGGC containing a more common C32-A38 pair was able to decode all cognate and near-cognate codons tested. We propose that many of the phylogenetically conserved sequence elements present in each tRNA have evolved to suppress translation of near-cognate codons.

Bacterial regulatory RNAs require the chaperone protein Hfq to enable their pairing to mRNAs. Recent data showed that there is a hierarchy among sRNAs in the competition for access to Hfq, which could be important for the tuning of sRNA-dependent translation regulation. Here, seven structurally different sRNAs were compared using filter-based competition assays. Moreover, chimeric sRNA constructs were designed to identify structure elements important for competition performance. The data showed that besides the 3′-terminal oligouridine sequences also the 5′-terminal structure elements of sRNAs were essential for their competition performance. When the binding of sRNAs to Hfq mutants was compared, the data showed the important role of the proximal and rim sites of Hfq for the binding of six out of seven sRNAs. However, ChiX sRNA, which was the most efficient competitor, bound Hfq in a unique way using the opposite—distal and proximal—faces of this ring-shaped protein. The data indicated that the simultaneous binding to the opposite faces of Hfq was enabled by separate adenosine-rich and uridine-rich sequences in the long, single-stranded region of ChiX. Overall, the results suggest that the individual structural composition of sRNAs serves to tune their performance to different levels resulting in a hierarchy of sRNAs in the competition for access to the Hfq protein.