SummaryThe conserved RNA-binding protein Hfq, originally discovered in Escherichia coli as a host factor for Q b b b b replicase, has emerged as a pleiotropic regulator that modulates the stability or the translation of an increasing number of mRNAs. During the past 5 years, Hfq-mediated control has been an area of increasing focus because the protein has been linked to the action of many versatile RNA-based regulators that use basepairing interactions to regulate the expression of target mRNAs. The recent findings that Hfq assists in bimolecular RNA-RNA interactions and is similar structurally and functionally to eukaryotic Sm proteins have further fuelled interest in this important post-transcriptional regulator. Here, we summarize the history of Hfq and highlight results that have led to an important gain in insight into the physiology, biochemistry and evolution of Hfq and its homologues.

SummaryThe Escherichia coli ompA mRNA, encoding a highly abundant outer membrane protein, has served as a model for regulated mRNA decay in bacteria. The halflife of this transcript correlates inversely with the bacterial growth rate and is growth stage-dependent. The stability of the messenger is determined by the 5 ′ ′ ′ ′ -untranslated region which possesses cleavage sites for RNase E. Hfq binds to this region, is essential for controlling the stability and has been suggested to directly regulate ompA mRNA decay. Here we report that the 78 nucleotide SraD RNA, which is highly conserved among Enterobacteriaceae, acts in destabilizing the ompA transcript when rapidly grown cells enter the stationary phase of growth. During this growth-stage the expression of SraD RNA becomes strongly increased. The SraD-mediated decay of ompA mRNA depends on Hfq and in vitro studies revealed that Hfq facilitates binding of the regulatory RNA to the translational initiation region of the messenger. Deletion of sraD , however, does not significantly affect the stability of the ompA mRNA in slowly growing cells. Our results indicate that distinct regulatory circuits are responsible for growth phase-and growth rate-dependent control of the ompA mRNA stability.

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