FIG. 5.
A model to account for trapping of the switch in the on orientation following DNA relaxation. The relevant features of the fim switch are shown: inverted repeats IRL and IRR, Lrp sites 1 and 2 (numbered hexagons), the PfimA promoter, and a putative reference point (Ref) located in the noninvertible region of the chromosome adjacent to IRL. Communication between Ref and the occupied Lrp binding sites facilitates the formation of an inhibitory complex (gray box). Negative supercoiling of the DNA assists extrusion of IRL from the inhibitory complex (top). Removal of negative superhelicity following gyrase inhibition abrogates extrusion of IRL, and the switch is trapped in the on orientation (middle). Elimination of Lrp binding disrupts the inhibitory complex, and rapid on-to-off switching occurs (bottom).