Hfq (HE1) stimulates ompA mRNA decay by interfering with ribosome binding

The adaptation of mRNA stability to environmental changes is a means of cel ls to adjust the level of gene expression. The Escherichia coli ompA mRNA h as served as one of the paradigms for regulated mRNA decay in prokaryotes. The stability of the transcript is known to be correlated inversely with th e bacterial growth rate. Thus, the regulation of ompA mRNA stability meets the physiological needs to adjust the level of ompA expression to the rate of cell division. Recently, host factor I (Hfq/HF1) was shown to be involve d in the regulation of ompA mRNA stability under slow growth conditions. He re, we present the first direct demonstration that 30S ribosomes bound to t he ompA 5'-UTR protect the transcript from RNase E cleavage in vitro. Howev er, the 30S protection was found to be abrogated in the presence of Hfq. To eprinting and in vitro translation assays revealed that translation of ompA is repressed in the presence of Hfq. These in vitro studies are corroborat ed by in vivo expression studies demonstrating that the reduced synthesis r ate of OmpA effected by Hfq results in functional inactivation of the ompA mRNA. The data are discussed in terms of a model wherein Hfq regulates the stability of ompA mRNA by competing with 30S ribosomes for binding to the o mpA 5'-UTR.