FIGURE 10.

Switching at the pap promoter region. Lrp binds co-operatively to either Lrp binding sites 1–3 (in the OFF-state, non-piliated) or to the Lrp binding sites 4–6 (ON-state, piliated). Lrp binding site 3 overlaps the papB promoter and Lrp binding to site 3 inhibits transcription. Lrp binding sites 2 and 5 overlaps with GATC sites and Lrp binding to either site prevents methylation of that site by DamMT. Lrp binding to sites 1–3 mutually excludes binding to sites 4–6. When in OFF-state, each DNA replication produce one hemimethylated GATC site (in Lrp site 5) and one unmethylated GATC site (in Lrp site 2) and dissociate Lrp from its binding sites. The OFF-state is preserved by rebinding of Lrp to the same binding sites around the unmethylated Lrp binding site. A shift from phase OFF to ON may occur if PapI mediates Lrp binding to the hemimethylated Lrp site 5, followed by Lrp binding to sites 4 and 6. The shift is further stabilized by full methylation of site 2 by DamMT and conversion of the hemimethylated site 5 to unmethylated by subsequent DNA replications. This figure is adapted from [275] with permission from Cell Press.