Fishing for Fluke: the Genetics of Flk and the Flagellar Type 3 Secretion Specificity Switch

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In the spring of 1991, using a simple genetic selection, the author and his colleagues discovered two negative regulators of flagellar gene expression, Flk and FlgM. They discovered that the negative regulator, FlgM, would sense the completion of the flagellar hook-basal body (HBB) and allow for transcription of filament genes, fliC or fljB, only after HBB completion. Upon HBB completion, FlgM became a substrate for secretion from the cell, and its negative regulatory activity was physically removed. The fliC::Mud--lac and fljB::Mud--lac fusions provided a nice, visual assay for Salmonella flagellar phase variation using lactose indicator medium. This chapter describes flagellar phase variation, flagellum assembly, and coupled gene regulation. When a fliC::MudJ insertion was introduced into HBB-defective single mutants, flgA through flgG, the cells were Lac-, but when the fliC::MudJ was introduced into a deletion of flgA through flgG, the cells were Lac+. Flk exerts its control over the flagellar assembly process at the level of the flagellar secretion specificity switch. The frequency of switching to late secretion in the absence of Fluke varied among the different rod-ring-hook mutant strains.