Acts as an anti-CsrA protein, binds CsrA and prevents it from repressing translation of its target genes, one of which is flagellin. Binds to flagellin (hag), which is implicated in polymerization, and participates in the assembly of the flagellum (PubMed:16936039). An antagonist to translational regulator CsrA, it binds CsrA at an allosteric site and non-competitively inhibits CsrA binding to hag RNA (PubMed:27516547). 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 translation regulator 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). Binds to CsrA and displaces it from hag mRNA (PubMed:21895793, PubMed:27516547). Binds to hag mRNA itself, but only at much higher concentrations than those required to displace CsrA (PubMed:21895793, PubMed:27516547). Interacts with flagellin in a 1:1 complex (PubMed:16936039, PubMed:21895793). Two molecules interact with each CsrA dimer; cannot interact with both flagellin and CsrA simultaneously (PubMed:21895793, PubMed:27516547). Has a higher affinity for CsrA than for flagellin (PubMed:21895793). Interacts directly with flagellin (hag), forms a 3-way complex of Hag, FliS and FliW in which Flis and FliW do not directly interact (PubMed:23144244). Interaction with Hag may occur via the C-terminus of Hag (PubMed:16936039). Part of the SigD-controlled yvyF-csrA operon and the SigA-controlled fliW-csrA operon (PubMed:17555441). Greatly reduced swarming motility, less flagellin (PubMed:16936039, PubMed:21895793). Fewer, shorter flagella assemble (PubMed:21895793). Loss of motility and flagellar assembly are suppressed by deletion of csrA (PubMed:21895793). Decreased expression of flagellin (Hag) which is suppressed by deletion of csrA (PubMed:23144244). Belongs to the FliW family.