Figure 6.

Model for a second role of dam in cSDR. (A) Shown schematically is a MutHLS-generated double strand break (arrowhead) in one of the daughter duplexes immediately behind a replisome that is progressing (ochre arrow) from oriC to Ter on the clockwise replichore in a dam mutant cell. (5′ to 3′ polarity for top strand in each of the duplexes is from left to right.) (B) Following RecBCD-mediated DNA degradation (shown by interrupted lines) from both ends of the break (64), the ori-proximal DSE that remains participates in repair by homologous recombination (green rectangle) with its sister duplex; alternatively (not shown), such recombination can occur also with a ‘cousin’ duplex (50,83). (C) One intermediate in the pathway of recombinational repair is shown, with the pair of mauve arrows marking sites of RuvC-mediated Holliday junction resolution. (D) Disposition of DNA strands following normal replication restart, with DnaB helicase (not shown) translocating from left to right on top strand of the parental duplex, towards Ter. (E) Disposition of DNA strands following reverse restart; PriA-mediated DnaB helicase loading is postulated to have occurred on strand shown in red (this panel, and panel C) with its translocation proceeding from right to left, towards oriC. The step shown by the question mark, in the progression from panel C to panel E, is to indicate that the intermediates in the reverse restart pathway are at present undefined. (F) Disposition of DNA strands and of three replisomes following homologous recombination (at site marked by green rectangle in panel E) and normal restart, similar to pathway depicted in panels B through D.