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. Author manuscript; available in PMC: 2015 Dec 1.
Published in final edited form as: Mol Microbiol. 2014 Oct 31;94(6):1272–1284. doi: 10.1111/mmi.12828

Figure 1.

Figure 1

The flagellar three class regulatory cascade. Proteins encoded by flhDC, the sole class I operon, form a complex required to activate class II gene transcription. Class II genes encode structural proteins of the platform that anchors the flagellum within the bacterial cell envelope (the basal body) and of the flexible linker that connects the platform with the filament (the hook), as well as the regulators fliZ and fliA. FliZ enhances class II gene expression, forming a positive feedback loop. Two mechanisms have been proposed to account for this loop. FliZ represses the class II repressor ydiV (i); thus fliZ and ydiV are mutually repressing repressors with opposing effects upon class II transcription. FliZ has also been shown to augment the level of the class I protein FlhC (ii). FliA is the sigma factor required for transcription of class III genes, including the flagellin filament protein FliC. The fliAZ operon is transcribed from both a class II (iii) and a class III (v) promoter, thus forming another positive feedback loop (the sigma factor activates transcription of its own operon).