Hfq is a bacterial RNA binding protein that facilitates small RNA-mediated posttranscriptional gene regulation. In Vibrio cholerae, Hfq and four Hfq-dependent small RNAs are essential for the expression of virulence genes, but little is known about this mechanism at the molecular level. To better understand V. cholerae Hfq structure and mechanism, we characterized the protein, alongside Escherichia coli Hfq for comparison, using biochemical and biophysical techniques. The N-terminal domain (NTD) of the two proteins is highly conserved, but the C-terminal regions (CTRs) vary in both sequence and length. Small-angle X-ray scattering studies showed that both proteins adopt a star-shaped hexameric structure in which the conserved NTD adopts the expected Sm fold while the variable CTR is disordered and extends radially outwards from the folded core. Despite their structural similarity, SDS-PAGE stability assays and collision-induced dissociation mass spectrometry revealed that the V. cholerae hexamer is less stable than that of E. coli. We propose that this is due to minor differences between the intersubunit interface formed by the NTDs and the ability of the E. coli CTR to stabilize this interface. However, based on electrophoretic mobility shift assays, the divergent CTRs do appear to perform a common function with regard to RNA-binding specificity. Overall, the similarities and differences in the fundamental properties of V. cholerae and E. coli Hfq provide insight into their assembly and molecular mechanisms.

OxyS and RprA are two small noncoding RNAs (sRNAs) that modulate the expression ofrpoS,encoding an alternative sigma factor that activates transcription of multipleEscherichia colistress-response genes. While RprA activatesrpoSfor translation, OxyS down-regulates the transcript. Crucially, the RNA binding protein Hfq is required for both sRNAs to function, although the specific role played by Hfq remains unclear. We have investigated RprA and OxyS interactions with Hfq using biochemical and biophysical approaches. In particular, we have obtained the molecular envelopes of the Hfq–sRNA complexes using small-angle scattering methods, which reveal key molecular details. These data indicate that Hfq does not substantially change shape upon complex formation, whereas the sRNAs do. We link the impact of Hfq binding, and the sRNA structural changes induced, to transcript stability with respect to RNase E degradation. In light of these findings, we discuss the role of Hfq in the opposing regulatory functions played by RprA and OxyS inrpoSregulation.

RNA turnover is an essential element of cellular homeostasis and response to environmental change. Whether the ribonucleases that mediate RNA turnover can respond to cellular metabolic status is an unresolved question. Here we present evidence that the Krebs cycle metabolite citrate affects the activity of Escherichia coli polynucleotide phosphorylase (PNPase) and, conversely, that cellular metabolism is affected widely by PNPase activity. An E. coli strain that requires PNPase for viability has suppressed growth in the presence of increased citrate concentration. Transcriptome analysis reveals a PNPase-mediated response to citrate, and PNPase deletion broadly impacts on the metabolome. In vitro, citrate directly binds and modulates PNPase activity, as predicted by crystallographic data. Binding of metal-chelated citrate in the active site at physiological concentrations appears to inhibit enzyme activity. However, metal-free citrate is bound at a vestigial active site, where it stimulates PNPase activity. Mutagenesis data confirmed a potential role of this vestigial site as an allosteric binding pocket that recognizes metal-free citrate. Collectively, these findings suggest that RNA degradative pathways communicate with central metabolism. This communication appears to be part of a feedback network that may contribute to global regulation of metabolism and cellular energy efficiency.

In Vibrio cholerae , the RNA binding protein and chaperone Hfq (VcHfq) facilitates the pairing of the quorum regulatory RNA (Qrr) small regulatory RNAs (sRNAs) to the 5′ untranslated regions of the mRNAs for a number of global regulators that modulate the expression of virulence genes. This Qrr-mediated sRNA circuit is an attractive antimicrobial target, but characterization at the molecular level is required for this to be realized. Here, we investigate the interactions between VcHfq and the Qrr sRNAs using a variety of biochemical and biophysical techniques. We show that the ring-shaped VcHfq hexamer binds the Qrrs with 1:1 stoichiometry through its proximal face, and the molecular envelope of the VcHfq-Qrr complex is experimentally determined from small angle scattering data to present the first structural glimpse of a Hfq-sRNA complex. This structure reveals that the VcHfq protein does not change shape on complex formation but the RNA does, suggesting that a chaperone role for VcHfq is a critical part of the VcHfq-Qrr interaction. Overall, these studies enhance our understanding of VcHfq-Qrr interactions.

Ribonucleases play essential roles in all aspects of RNA metabolism, including the coordination of post-transcriptional gene regulation that allows organisms to respond to internal changes and environmental stimuli. However, as inherently destructive enzymes, their activity must be carefully controlled. Recent research exemplifies the repertoire of regulatory strategies employed by ribonucleases. The activity of the phosphorolytic exoribonuclease, polynucleotide phosphorylase (PNPase), has previously been shown to be modulated by the Krebs cycle metabolite citrate in Escherichia coli. Here, we provide evidence for the existence of citrate-mediated inhibition of ribonucleases in all three domains of life. In silico molecular docking studies predict that citrate will bind not only to bacterial PNPases from E. coli and Streptomyces antibioticus, but also PNPase from human mitochondria and the structurally and functionally related archaeal exosome complex from Sulfolobus solfataricus. Critically, we show experimentally that citrate also inhibits the exoribonuclease activity of bacterial, eukaryotic and archaeal PNPase homologues in vitro. Furthermore, bioinformatics data, showing key citrate-binding motifs conserved across a broad range of PNPase homologues, suggests that this regulatory mechanism may be widespread. Overall, our data highlight a communicative link between ribonuclease activity and central metabolism that may have been conserved through the course of evolution.

MicA is a small non-coding RNA that regulates ompA mRNA translation in Escherichia coli . MicA has an inhibitory function, base pairing to the translation initiation region of target mRNAs through short sequences of complementarity, blocking their ribosome-binding sites. The MicA structure contains two stem loops, which impede its interaction with target mRNAs, and it is thought that the RNA chaperone protein Hfq, known to be involved in MicA regulation of ompA , may structurally remodel MicA to reveal the ompA- binding site for cognate pairing. To further characterize these interactions, we undertook biochemical and biophysical studies using native MicA and a ‘stabilized’ version, modified to mimic the conformational state of MicA where the ompA -binding site is exposed. Our data corroborate two proposed roles for Hfq: first, to bring both MicA and ompA into close proximity, and second, to restructure MicA to allow exposure of the ompA -binding site for pairing, thereby demonstrating the RNA chaperone function of Hfq. Additionally, at accumulated MicA levels, we identified a Mg ²⁺ -dependent self-association that occludes the ompA -recognition region. We discuss the potential contribution of an Mg ²⁺ -mediated conformational switch of MicA for the regulation of MicA function.