TRANSCRIPTIONAL ANALYSIS OF THE FLGK AND FLID OPERONS OF SALMONELLA-TYPHIMURIUM WHICH ENCODE FLAGELLAR HOOK-ASSOCIATED PROTEINS

In Salmonella typhimurium, three hook-associated proteins, HAP1, HAP2 and HAP3, are known to be essential for formation of flagellar filamen t. HAP1 and HAP2 are encoded by the flgK and flgL genes, respectively, which together constitute an operon, called the flgK operon. HAP3 is encoded by the fliD gene which forms part of the fliD operon together with the fliS and fliT genes. In the flagellar regulon, the operons ar e divided into three classes, 1, 2 and 3, based on their positions wit hin a transcriptional hierarchy. Transcriptional analysis suggested th at the flgK and fliD operons should belong to class 3, whose expressio n is dependent on the flagellum-specific sigma factor FliA. However, b iochemical data indicated that these HAP proteins are detectable even in the hook-basal body structures produced by the fliA mutant. This wo rk was carried out to resolve this discrepancy. More careful examinati on of transcription revealed that the PIA mutation reduces but does no t eliminate the expression of these operons, whereas a mutation in the flhD operon, which encodes activator proteins for the class 2 operons , eliminates their expression. This suggests that the flgK and fliD op erons may be transcribed from both class 2 and class 3 promoters. Prim er extension analysis indicated that the promoter region of fliD conta ins both class 2 and class 3 promoters, while that of flgK contains on ly a class 3 promoter. Transposon insertion into the flgB operon, whic h belongs to class 2 and lies upstream of the flgK operon, was found t o decrease the expression of the flgK operon to the basal level. This indicates that the flgK operon is transcribed both from its own class 3 promoter and from the class 2 promoter that controls the upstream fl gB operon.