3.1) Hfq, a RNA chaperone
The similarity between RNA-binding Sm proteins and Hfq has led to many studies focusing on the interactions of Hfq with RNA. E. coli Hfq has been reported to bind to the sRNAs oxyS (Zhang, 1998),rprA (Wassarman et al. , 2001) and dsrA (Sledjeski, Whitman and Zhang, 2001) and to modulate the translation of rpoS (Battesti, Majdalani and Gottesman, 2011). Considering that RpoS is a sigma factor expressed under various stress conditions, it was assumed that most of the reported phenotypes of the hfq mutants were due to Hfq mediating the stress response through the interaction betweenrpoS mRNA and sRNAs. Accordingly, the hfq mutant shows a strongly reduced expression of representative RpoS-regulated genes (Muffler et al. , 1997). Since these early studies, Hfq has been reported to facilitate base-pairing between a large number oftrans -encoded sRNAs and their mRNA targets (Holmqvist et al. , 2016). In general, the U-rich RNA sequences, usually corresponding to the 3’-end of sRNAs, bind to the proximal face of the toroid, whereas A-rich sequences, mostly found in mRNAs, bind to the distal face of Hfq (Figure 1 ). In addition, the lateral rim binds U-rich sequences found in sRNAs (Sauer, Schmidt and Weichenrieder, 2012). Thus, Hfq’s three faces allow it to bind two different RNA strands simultaneously, making it an effective RNA chaperone that brings the regulatory RNA and its mRNA target closer together. Several outcomes are possible following the sRNA:mRNA hybridisation : (i) prevention or promotion of translation by concealing or facilitating access to the ribosome binding site, respectively (Maki et al. , 2008; Fröhlich and Vogel, 2009); (ii) prevention or facilitation of RNase degradation of the target mRNA (Mollet al. , 2003; Mohanty, Maples and Kushner, 2004). Taken together, the role of Hfq in post-transcriptional regulation is crucial. For more on the different regulatory mechanisms exhibited by Hfq, see Vogel and Luisi, 2011.