<p> This entry represents the RNA-binding pleiotropic regulator Hfq, a small, Sm-like protein of bacteria. It helps pair regulatory non-coding RNAs with complementary mRNA target regions. It enhances the elongation of poly(A) tails on mRNA. It appears also to protect RNase E recognition sites (A/U-rich sequences with adjacent stem-loop structures) from cleavage. Being pleiotropic, it differs in some of its activities in different species. Hfq binds the non-coding regulatory RNA DsrA (see Rfam:RF00014) in the few species known to have it: <taxon tax_id="562">Escherichia coli</taxon>, <taxon tax_id="623">Shigella flexneri</taxon>, Salmonella spp. In <taxon tax_id="7">Azorhizobium caulinodans</taxon>, an hfq mutant is unable to express nifA, and Hfq is called NrfA, for nif regulatory factor (see <cite idref="PUB00016696"/>). The name Hfq reflects phenomenology as a host factor for phage Q-beta RNA replication.</p><p>The Hfq protein is conserved in a wide range of bacteria and varies in length from 70 to 100 amino acids. In all cases, a conserved Sm motif is located in the N-terminal halves of the molecules. The Hfq protein of E. coli is an 11 kDa polypeptide that forms a hexameric ring-shaped structure. Structural studies have suggested that the beta-4 strand in one molecule dimerises with the beta-5 strand of a neighbouring subunit to form the hexamer. These two strands move with a concerted mobility which may explain the stability of the entire structure [<cite idref="PUB00007236"/>].</p><p>The architecture of the Hfq-RNA complex suggests two, not mutually exclusive, mechanisms by which Hfq might exert its function as modulator of RNA-RNA interactions. First, when Hfq binds single-stranded RNA, the target site is unwound in a circular manner. This would greatly destabilise surrounding RNA structures that are located several nucleotides on either side of the binding site, thereby permitting new RNA-RNA interactions. Secondly, the repetition of identical BPs on the Hfq hexamer implies that the binding surface can accommodate more than just a single RNA target. This would allow simultaneous binding of two RNA strands and could greatly enhance interaction between the strands [<cite idref="PUB00014979"/>].</p> Host factor Hfq