Figure 7.

Model for formation of 2 μm rolling circle replication intermediate via BIR. (a) Flp (blue) engaged in a covalent intermediate with DNA at FRT is encountered by one or both replication forks (RFs), resulting in (b) a species containing a DSB where one end has Flp covalently linked to the 3′ end, whereas the other 3′ end is free. (c) The HRR pathway catalyzes invasion of the free 3′ end into an intact circular copy of 2 μm, generating a new RF. Depending on the mechanism by which BIR occurs (there are several models), the RF could be followed by a Holliday junction (HJ) (shown) or only the 3′end/leading strand could invade, forming a D loop (not shown). (d) Progression of the RF, accompanied by branch migration of the HJ (shown) or progression of the D loop followed by separate lagging strand synthesis (not shown) would give rise to rolling circle replication. (e) If the RF progressed more rapidly than the HJ, it would replicate the circular template until RF progression is impeded by the HJ. RF progression might then promote branch migration of the HJ. Red lines indicate DNA synthesized by the BIR-derived RF. Curved arrows indicate direction of rotation of rolling circle.