| Summary: | A motile but non-swarming transposon mutant of <I>P. mirabilis</I> was phenotypically characterised and found to be defective in cell elongation and hyperexpression of cell surface flagella and HpmA toxin. The transposon had inserted into the 441bp <I>flgN</I>, a flagella gene of undefined function, cotranscribed in an operon with the Class 3 anti-σ<SUP>28</SUP> gene <I>flgM</I>. Flagella gene transcription was however not compromised in the mutant and as Class 3 genes were not strongly downregulated by FlgM feedback, the defect did not appear to lie in the membrane export machinery. Loss of FlgN reduced incorporation of flagellin into the membrane and caused release of unpolymerised FliC into the extracellular medium. The data suggest that FlgN facilitates acquisition of flagellin into filaments, a role that may be especially critical in attaining the threshold level of flagella required for swarming. <I>Proteus</I> FlgN was found to have leucine zipper-like motifs arranged on potential amphipathic helices, a feature conserved in cytosolic chaperones for the exported substrates of flagella-related type III virulence export systems. Expression of HpmA was investigated in wildtype <I>P. mirabilis </I>and in the <I>flgN</I> and <I>FlhA</I> flagellar gene mutants, both of which exhibit reduced swarm-specific haemolytic activity. Primer extension analysis of <I>hpmBA</I> genes revealed a differentially regulated σ<SUP>70</SUP> promoter upstream of <I>hpmB</I> and several potential transcription initiation or RNA processing sites upstream of <I>hpmA</I>. Northern blot analyses of full length, truncated and deleted transcripts indicated processing of the unstable full length <I>hpmBA</I> transcript to yield a stable <I>hpmA</I> transcript. Transcriptional fusions of <I>hpmB</I> and <I>hpmA</I> to the <I>luxAB</I> reporter genes strengthened the view that swarm-specific regulation of transcription initiation was centred on the region 5' of <I>hpmB,</I> and involved sequences 5' of the σ<SUP>70</SUP> promoter. The fusion analyses also indicated that repression of <I>hpmBA</I> transcription in the flagellar mutants could be relieved by truncation of the <I>hpmB</I> upstream region.
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