RNA chaperone that binds small regulatory RNA (sRNAs) and mRNAs to facilitate mRNA translational regulation in response to envelope stress, environmental stress and changes in metabolite concentrations. Also binds with high specificity to tRNAs (By similarity). Positively regulates the expression of the yst gene for heat-stable enterotoxin (Y-ST). Homohexamer. Belongs to the Hfq family.