3) Hfq: a double function protein?
Hfq is an 11 kDa homohexameric protein present in approximately half of sequenced bacterial genomes and in some archaeal species (Table 1 ) (Sun, 2002; Mura et al. , 2013). It was first described as a host factor required for the replication of the bacteriophage Qβ RNA (Franze de Fernandez, Hayward and August, 1972). An E. coli cell contains approximately 10,000 Hfq hexamers, making it one of the most abundant proteins. The concentration of Hfq in the stationary phase is controversial in the literature. Although western blot quantification by Azam and Ishihama, (1999) showed a decrease in Hfq protein during the stationary phase of growth, other studies agree on a twofold increase in the protein level during the stationary phase of growth (Tsui, Feng and Winkler, 1997; Diestra et al. , 2009). 10%-20% of these hexamers are found in the nucleoid, 50% are membrane-bound fractions and 30% are localised in the cytoplasm (Azam, Hiraga and Ishihama, 2000; Diestraet al. , 2009). A Hfq monomer is composed of two domains named the N-terminal region (NTR) and the C-terminal region (CTR) (Figure 1 ). The NTR is highly conserved across bacteria and in E. coli, it is a 65 amino acid long peptide that is structurally related to the eukaryotic Sm family of proteins (Mura et al. , 2013). The Sm proteins have been shown to participate in nucleic acid-related processes, including splicing, telomere replication, RNA decapping and decay (Wilusz and Wilusz, 2013). The structure of the NTR reveals that it folds into five β-sheets and one N-terminal α-helix. Six monomers assemble into a typical toroidal hexameric ring (Figure 1 ) (Sauter, 2003). This toroid can be divided into three parts: the proximal face, where the α-helix is located, the distal face where most of the β-sheets are exposed to the solvent and the lateral rim. The CTR is located at the periphery of the toroid, is predicted to be unstructured and has been reported to be present only in a subset of Gram-negative bacteria (Vogel and Luisi, 2011). Disruption of thehfq gene in most bacterial species, including E. coli , results in diverse phenotypes, highlighting the pleiotropic functions of this protein. These include reduced growth rate, virulence, motility and biofilm formation, as well as modifications in oxidative and osmotic sensitivity (Tsui, Leung and Winkler, 1994; Sittka et al. , 2007; Leonard et al. , 2021). However, the requirement for Hfq appears to be facultative in a few bacteria, most of which are Gram-positive, such as Bacillus subtilis or Staphylococcus aureus (Jousselin, Metzinger and Felden, 2009) .