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The RNA chaperone Hfq plays a critical role in sRNA-mediated gene regulation in enteric bacteria. The major role of Hfq is to stimulate base-pairing between sRNAs and target mRNAs by binding both RNAs through three RNA-binding surfaces. To understand the post-transcriptional network exerted by Hfq and its associated sRNAs, it is important to know how the cellular concentration of Hfq is regulated. While an early study showed that  hfq  translation is repressed by Hfq, the detailed mechanism and biological significance of the  hfq  autoregulation remain to be studied. Here, we show that the synthesis of Hfq is strictly autoregulated to maintain the cellular concentration of Hfq within a limited range even when the  hfq  mRNA is overexpressed from a plasmid-borne  hfq  gene. Mutational and biochemical studies demonstrate that Hfq represses its own translation primarily by binding to the  hfq  mRNA through the distal face. The growth of cells harboring the  hfq  plasmid is markedly inhibited due to an increased Hfq level when the distal face of Hfq is mutated or the 5′-UTR of  hfq  is mutated. A mutation in the rim suppresses the growth inhibition caused by the distal face mutation, suggesting that the interaction of Hfq with undefined RNAs through the rim is responsible for the growth inhibition by the increased Hfq level. In addition, the data suggest that the  hfq  autoregulation operates not only in cells harboring a multicopy  hfq  gene but also in the wild-type cells.

Mechanism and physiological significance of autoregulation of the Escherichia coli hfq gene