Figure 5.

A model of P1 plasmid replication requiring replication-induced boost of repA transcription (curved arrows) synthesis to saturate origin iterons with RepA (●). The model shows that handcuffing occurs with unsaturated daughter origins which shuts off repA transcription. The transcription therefore precedes handcuffing. Handcuffing is effectively reversed by increase in cell volume and increase in RepA concentration because of chaperone-mediated activation of newly synthesized RepA. When extra iterons are provided by using foreign vectors, handcuffing prevents new RepA synthesis and, hence, origin saturation. The replication under these conditions therefore depends on an external source of RepA.