FliZ is a post-translational activator of FlhD₄C₂-dependent flagellar gene expression

Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 61801; Centre for Bacterial Cell Biology, Newcastle University, Framlington Place, Newcastle upon Tyne, United Kingdom, NE2 4HH; Institute for Cell and Molecular Biosciences, Newcastle University, Framlington Place, Newcastle upon Tyne, United Kingdom, NE2 4HH

^* To whom correspondence should be addressed. Email: p.d.aldridge{at}ncl.ac.uk. chris{at}scs.uiuc.edu.

	Abstract

Flagellar assembly proceeds in a sequential manner beginning at the base and concluding with the filament. A critical aspect of assembly is that gene expression is coupled to assembly. When cells transition from a non-flagellated to a flagellated state, gene expression is sequential, reflecting the manner in which the flagellum is made. A key mechanism for establishing this temporal hierarchy is the ²⁸-FlgM checkpoint, which couples the expression of late flagellar (P_class3) genes to the completion of the hook-basal body. In this work, we investigate the role of FliZ in coupling middle flagellar (P_class2) gene expression to assembly in Salmonella enterica serovar Typhimurium. We demonstrate that FliZ is an FlhD₄C₂-dependent activator of P_class2/middle gene expression. Our results suggest that FliZ regulates the concentration of FlhD₄C₂ post-translationally. We also demonstrate that FliZ functions independently of the flagellar-specific sigma factor ²⁸ and the filament-cap chaperone/FlhD₄C₂-inhibitor FliT. Furthermore, we show that the previously described ability of ²⁸ to activate P_class2/middle gene expression is, in fact, due to FliZ, as both are expressed from the same overlapping P_class2 and P_class3 promoters at the fliAZY locus. We conclude by discussing the role of FliZ regulation with respect to flagellar biosynthesis based on our characterization of gene expression and FliZ's role in swimming and swarming motility.