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 genetic information to the ability to sense the environment and move towards or away from specific stimuli using flagella. In Escherichia coli and related species, motility is regulated by several global regulatory circuits, which converge to modulate the overall expression of the master operon for flagellum biosynthesis, flhDC. We now show that the global regulator CsrA of E. coli 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 fourfold higher levels in csrA wild-type strains than in isogenic csrA mutants. Purified recombinant CsrA protein stimulated the coupled transcription-translation of flhDC′-′lacZ in S-30 extracts and bound to the 5′ segment of flhDC mRNA in RNA mobility shift assays. The steady-state level of flhDC mRNA was higher and its half-life was ≈threefold greater in a csrA wild-type versus a csrA mutant strain. Thus, CsrA stimulates flhDC gene expression by a post-transcriptional mechanism reminiscent of its function in the repression of glycogen biosynthesis.