The flagellar hook protein, FlgE, of Salmonella enterica serovar typhimurium is posttranscriptionally regulated in response to the stage of flagellarassembly

We investigated the posttranscriptional regulation of flgE, a class 2 gene that encodes the hook subunit protein of the flagella. RNase protection ass ays demonstrated that the flgE gene was transcribed at comparable levels in numerous strains defective in known steps of flagellar assembly. However, Western analyses of these strains demonstrated substantial differences in F lgE protein levels. Although wild-type FlgE levels were observed in strains with deletions of genes encoding components of the switch complex and the flagellum-specific secretion apparatus, no protein was detected in a strain with deletions of the rod, ring, and hook-associated proteins. To determin e whether FlgE levels,were affected by the stage of hook-basal-body assembl y. Western analysis was performed on strains with mutations at individual l oci encompassed by the deletion. FlgE protein was undetectable in rod mutan ts, intermediate in ring mutants, and wild type in hook-associated protein mutants. The lack of negative regulation in switch complex and flagellum-sp ecific secretion apparatus deletion mutants blocked for flagellar construct ion prior to rod assembly suggests that these structures play a role in the negative regulation of FlgE. Quantitative Western analyses of numerous fla gellar mutants indicate that FlgE levels reflect the stage at which flagell ar assembly is blocked. These data protide evidence for negative posttransc riptional regulation of FlgE in response to the stage of flagellar assembly .