Positive regulation of motility and flhDC expression by the RNA-binding protein CsrA of Escherichia coli

Many species of bacteria devote considerable metabolic resources and geneti c information to the ability to sense the environment and move towards or a way from specific stimuli using flagella. In Escherichia coil and related s pecies, motility is regulated by several global regulatory circuits, which converge to modulate the overall expression of the master operon for flagel lum biosynthesis, flhDC, We now show that the global regulator CsrA of E. c oil K-12 is necessary for motility under a variety of growth conditions, as a result of its role as an activator of flhDC expression. A chromosomally encoded flhDC'-'lacZ translational fusion was expressed at three- to fourfo ld higher levels in csrA wild-type strains than in isogenic csrA mutants. P urified recombinant CsrA protein stimulated the coupled transcription-trans lation of flhDC'-' lacZ in S-30 extracts and bound to the 5' segment of flh DC mRNA in RNA mobility shift assays. The steady-state level of flhDC mRNA was higher and its half-life was approximate to threefold greater in a csrA wild-type versus a csrA mutant strain. Thus, CsrA stimulates flhDC gene ex pression by a post-transcriptional mechanism reminiscent of its function in the repression of glycogen biosynthesis.