HOST FACTOR-I, HFQ, BINDS TO ESCHERICHIA-COLI OMPA MESSENGER-RNA IN AGROWTH-RATE DEPENDENT FASHION AND REGULATES ITS STABILITY

The stability of the ompA mRNA depends on the bacterial growth rate. T he 5' untranslated region is the stability determinant of this transcr ipt and the target of the endoribonuclease, RNase E, the key player of mRNA degradation. An RNA-binding protein with affinity for the 5' unt ranslated region ompA was purified and identified as Hfq, a host facto r initially recognized for its function in phage Q beta replication. T he ompA RNA-binding activity parallels the amount of Hfq, which is ele vated in bacteria cultured at slow growth rate, a condition leading to facilitated degradation of the ompA mRNA. In hfq mutant cells with a deficient Hfq gene product, the RNA-binding activity is missing, and a nalysis of the ompA mRNA showed that the growth-rate dependence of deg radation is lost. Furthermore, the half-life of the ompA mRNA is prolo nged in the mutant cells, irrespective of growth rate. Hfq has no affi nity for the Ipp transcript whose degradation, like that of bulk mRNA is not affected by bacterial growth rate. Compatible with our results, we found that the intracellular concentration of RNase E and its asso ciated degradosome components is independent of bacterial growth rate. Thus our results suggest a regulatory role for Hfq that specifically facilitates the ompA mRNA degradation in a growth rate dependent manne r.