Completion of the hook-basal body complex of the Salmonella typhimurium flagellum is coupled to FlgM secretion and fliC transcription

The flhDC operon of Salmonella typhimurium is the master control operon req uired for the expression of the entire flagellar regulon. The flagellar mas ter operon was placed under the tetracycline-inducible promoter P-tetA usin g the T-POP transposon. Cells containing this construct are motile in the p resence of tetracycline and non-motile without inducer present. No flagella were visible under the electron microscope when cells were grown without i nducer. The class 1, class 2 and class 3 promoters of the flagellar regulon are temporally regulated, After addition of tetracycline, the class 1 flhD C operon was transcribed immediately. Transcription of flgM (which is trans cribed from both class 2 and class 3 promoters) began 15 min after inductio n. At 20 min after induction, the class 2 fliA promoter became active and i ntracellular FliA protein levels increased; at 30 min after induction, the class 3 fliC promoter was activated, Induction of fliC gene expression coin cides with the appearance of FlgM anti-sigma factor in the growth medium. T his also coincides with the completion of hook-basal body structures. Rolli ng cells first appeared 35 min after induction, and excess hook protein (Fl gE) was also found in the growth medium at this time. At 45 min after induc tion, nascent flagellar filaments became visible in electron micrographs an d over 40% of the cells exhibited some swimming behaviour. Multiple flagell a assemble and grow on individual cells after induction of the master opero n, These results confirm that the flagellar regulatory hierarchy of S. typh imurium is temporally regulated after induction. Both FlgM secretion and cl ass 3 gene expression occur upon completion of the hook-basal body structur e.