<i>Caulobacter crescentus</i> Hfq structure reveals a conserved mechanism of RNA annealing regulation.

Abstract

We have solved the X-ray crystal structure of the RNA chaperone protein Hfq from the alpha-proteobacterium <i>Caulobacter crescentus</i> to 2.15-Å resolution, resolving the conserved core of the protein and the entire C-terminal domain (CTD). The structure reveals that the CTD of neighboring hexamers pack in crystal contacts, and that the acidic residues at the C-terminal tip of the protein interact with positive residues on the rim of Hfq, as has been recently proposed for a mechanism of modulating RNA binding. De novo computational models predict a similar docking of the acidic tip residues against the core of Hfq. We also show that <i>C. crescentus</i> Hfq has sRNA binding and RNA annealing activities and is capable of facilitating the annealing of certain <i>Escherichia coli</i> sRNA:mRNA pairs in vivo. Finally, we describe how the Hfq CTD and its acidic tip residues provide a mechanism to modulate annealing activity and substrate specificity in various bacteria.