The many genes involved in flagellar structure and function in and are located in three major clusters on the chromosome: flagellar regions I, II and III. We have found that region III does not consist of a contiguous set of flagellar genes, as was thought, but that in there is almost 7 kb of DNA between the filament cap gene, , and the next known flagellar gene, ; a similar situation occurs in . Most of this DNA is unrelated to flagellar function, since a mutant in which 5.4 kb of it had been deleted remained fully motile and chemotactic as judged by swarming on semi-solid agar. We have therefore subdivided flagellar region III into two regions, IIIa and IIIb. The known genes in region IIIa are , all of which are involved in filament structure and assembly, while region IIIb contains genes , all of which are related to formation of the hook (basal-body)-complex or to even earlier assembly events. We have found that , the last known gene in region IIIa, is immediately followed by two additional genes, both necessary for flagellation, which we have designated and . They encode small proteins with deduced molecular masses of about 15 kDa and 14 kDa, respectively. The functions of FliS and FliT remain to be determined, but they do not appear to be members of the axial family of structural proteins to which FliD belongs.


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