A MOTILE BUT NON-SWARMING MUTANT OF PROTEUS-MIRABILIS LACKS FLGN, A FACILITATOR OF FLAGELLA FILAMENT ASSEMBLY

A TnphoA mutant of Proteus mirabilis was isolated, which had lost the ability to swarm, yet was still motile. The transposon had inserted in to flgN, a flagella gene encoding a 147-amino-acid protein of undefine d function. Proteus flgN is arranged in an operon with the class III a nti-sigma(2B) gene, flgM, flanked by the class II genes, flgA, flgBCD and flhBA, and a novel putative virulence-related gene, The flgN mutat ion caused a substantial reduction in cell surface-associated flagelli n, particularly during differentiation to the normally hyperflagellate d swarm cell. This was not due to an effect on flagella gene expressio n or a typical defect in the flagella export apparatus as there was no class III gene downregulation by FlgM feedback, or intracellular flag ellin accumulation. Loss of FlgN nevertheless caused a severe reductio n in the incorporation of pulse-labelled flagellin into the membrane/f lagellum fraction of differentiating cells. Substantial amounts of bot h non-oligomeric flagellin and flagellin degradation products appeared in the extracellular medium, although the few mature filaments made b y the mutant were no more sensitive to proteolysis than those of the w ild type, FlgN appeared soluble and active in the cytosol. The data su ggest that the function of FlgN is to facilitate the initiation of fla gella filament assembly, a role that may be especially critical in att aining the much higher concentration of surface flagellin required for swarming, Proteus FlgN has leucine zipper-like motifs arranged on pot ential amphipathic helices, a feature conserved in cytosolic chaperone s for the exported substrates of flagella-related type III virulence s ystems, while gel filtration of flgN from the soluble cell fraction di d not establish an interaction with flagellin, it indicated that FlgN may associate with an unknown component and/or form an oligomer.