Replication Fork Reversal after Replication–Transcription Collision
Figure 7
Model for the restart of forks arrested by a highly expressed, oppositely oriented rrn operon.
A blocked fork is either reversed (RFR) or re-replicated by a following round of replication initiated at the replication origin (re-replication). The product of RuvABC-catalyzed resolution of the HJ formed by fork reversal, and the product of re-replication are similar origin-proximal dsDNA ends (left part of the model). These DNA ends are repaired in Rec+ cells by homologous recombination catalyzed by RecBCD, RecA and RuvABC (not shown), but remain unrepaired in recBC and recA recD mutants, where they are detected by electrophoresis of Not1- or I-Sce1-treated DNA (Figure 3, Figure 4, Figure 5, Figure 6). In Rec+ cells the dsDNA end formed by fork reversal can be directly acted upon by RecBCD (see Figure 1) and processed by either homologous recombination (RecBC(D)-RecA-RuvABC pathway) or by DNA degradation (RecBCD (exo V) pathway). Reversed forks resetting by either pathway produces a replication fork that has moved backward, further from the obstacle than the original blocked fork. We propose that the reloading of new replisome at such forks favors the binding of a second accessory helicase (DinG or UvrD), required with Rep for replication across the inverted rrn operon.