Positive autoregulation of the flhDC operon in Proteus mirabilis

Using a variety of techniques, we demonstrate the Class I regulator of the flagellar cascade, FlhD4C2, can activate its own expression in Proteus mirabilis. This activation was direct, as the FlhD4C2 protein specifically bound to its promoter region. The expression of bacterial genes under a positive feedback control can exhibit varying levels between cells due to stochastic fluctuations that activate the feedback loop and result in some cells in an “ON” state. Cells containing a chromosomal flhDC::gfp transcriptional fusion exhibited a heterogeneous pattern of expression within the population during growth on agar surfaces and the percentage of cells expressing GFP increased as cells approached swarmer cell differentiation. Disrupting the FlhD4C2 -mediated positive feedback loop significantly reduced the frequency of cells exhibiting GFP expression.