A translational regulator that binds mRNA to regulate translation initiation and/or mRNA stability. Usually binds in the 5'-UTR at or near the Shine-Dalgarno sequence preventing ribosome-binding, thus repressing translation. Represses expression of flagellin (hag) in a post-transcriptional fashion. Specifically binds to 2 sites in the 5'-UTR of hag mRNA in a cooperative fashion; the second site overlaps the Shine-Dalgarno sequence and prevents 30S ribosomal subunit binding (PubMed:17555441). Mutation of either binding site abolishes CsrA regulation of hag expression (PubMed:17555441, PubMed:21895793). Repression is greater in the 1A96 than 168 genetic background and higher in minimal than rich medium (PubMed:17555441). Translation repression is antagonized by FliW (PubMed:21895793). Partner switching by flagellin between FliW and CsrA provides a flagellar assembly checkpoint to tightly control the timing of flagellin synthesis. Flagellin binds to assembly factor FliW, freeing CsrA to repress translation of the flagellin mRNA. When the flagellar hook is assembled flagellin is secreted, depleting intracellular flagellin, which frees FliW to interact with CsrA and inhibits CsrA binding to mRNA. This derepresses flagellin translation and provides protein for flagellar assembly. Once the flagellar filament is completed cytoplasmic flagellin levels rise and CsrA translation repression of flagellin reinitiates (PubMed:21895793, PubMed:27516547). Overexpression leads to a dramatic reduction in motility, a significant reduction in flagellin synthesis and reduced flagella assembly (PubMed:21895793). Homodimer (PubMed:17555441). The beta-strands of each monomer intercalate to form a hydrophobic core while the alpha-helices form wings that extend away from the core (By similarity). Two molecules of FliW interact with 1 homodimer (PubMed:21895793, PubMed:27516547). mRNA and FliW bind to different sites on CsrA (PubMed:27516547). Part of the SigD-controlled yvyF-csrA operon and the SigA-controlled fliW-csrA operon (PubMed:17555441). Expressed from the SigA operon at low levels during log phase, with a 5-fold increase as the culture transitions to stationary phase, peaking 1 hour later. Contacts FliW via its C-terminus (residues 49-58). Increased expression of flagellin (hag), 30% decrease in motility halo (PubMed:17555441). Suppresses the motility loss and flagellar assembly defect of an fliW deletion (PubMed:21895793). Suppresses the phenotypes associated with deletion of the intracellular flagella chaperone fliS (PubMed:23144244). Belongs to the CsrA/RsmA family.