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Negative feedback from a Proteus class II flagellum export defect to the flhDC master operon controlling cell division and flagellum assembly

The Proteus mirabilis flagellum class I flhDC operon was isolated, and its transcript was shown to originate from a sigma70 promoter 244 bp 5' of flhD and 29 bp 3' of a putative cyclic AMP receptor protein-binding site. Expression of this regulatory master operon increased strongly as cells differentiated into elongated hyperflagellated swarm filaments, and cell populations artificially overexpressing flhDC migrated sooner and faster. A class II flhA transposon mutant was reduced in flagellum class III gene expression, as would be expected from the FlgM anti-sigma28 accumulation demonstrated in Salmonella typhimurium, but was unexpectedly also reduced in cell elongation. Here, we show that levels of flhDC transcript were ca. 10-fold lower in this flagellum export mutant, indicating that in cells defective in flagellum assembly, there is additional negative feedback via flhDC. In support of this view, artificial overexpression of flhDC in the flhA mutant restored elongation but not class III flagellum gene transcription.