Positive autoregulation of the flhDC operon in Proteus mirabilis.

Using a variety of techniques, we demonstrate the Class I regulator of the flagellar cascade, FlhD4 C2 , can activate its own expression in Proteus mirabilis. This activation was direct, as the FlhD4 C2 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 FlhD4 C2 -mediated positive feedback loop significantly reduced the frequency of cells exhibiting GFP expression.