Characterization of the fliE genes of Escherichia coli and Salmonella typhimurium and identification of the FliE protein as a component of the flagellar hook-basal body complex

Within flagellar region III of Escherichia coli and Salmonella typhimurium, the genomic organization has been largely established. An exception is fliE, a gene whose exact location and product function are not well understood. We cloned the fliE gene, obtained its DNA sequence, and identified its product.fliE was found to be a monocistronic transcriptional unit, adjacent to and divergent from the large fliF operon. It is several kilobases distant from the nearest flagellar operon in the other direction, the fliD operon, and constitutes the first operon within the newly defined region IIIb, which contains the genes fliE through fliR.fliE encodes a small, moderately hydrophilic protein with a deduced molecular mass of 11,114 Da (E. coli) or 11,065 Da (S. typhimurium). We identified a protein within the isolated hook-basal body complex as the fliE gene product on the basis of its size and comparison of its N-terminal amino acid sequence with that deduced from the gene sequence. From gel electrophoresis and autoradiography of 35S-labeled S. typhimurium hook-basal body complexes (C.J. Jones, R.M. Macnab, H. Okino, and S.-I. Aizawa, J. Mol. Biol. 212:377-387, 1990) and the deduced number of sulfur-containing residues in FliE, we estimated the stoichiometry of the protein in the hook-basal body complex to be about nine subunits. FliE does not undergo cleavage of a signal peptide, nor does it show any sequence similarity to the axial components like the rod or hook proteins, which are believed to be exported by the flagellum-specific export pathway. On the basis of this and other evidence, we suggest that FliE may be in the vicinity of the MS ring, perhaps acting as an adaptor protein between the ring and rod substructures.