FIG. 5.

Model for topological chaos at dif and SOS and Rec-less degradation in a GyrB652 mutant. A. In strains with WT gyrase, initiation at oriC (0) leads to semiconservative (red strands) and bidirectional replication using two forks (labeled 1and 2). B. At 42°C in rich medium, rapid cell growth allows a second round of replication initiates at each oriC leading to four more forks (labeled 3, 4, 5, and 6) and new blue strands of DNA. C. As replication forks 1and 2 complete synthesis at the terminus, cell division proceeds with daughter cells inheriting two complete and two partially replicated chromosomes (not shown). When the second round is complete, four genome equivalents are produced with two black/blue strands and two red/blue strands. D. Strains with a GyrB652 gyrase have well organized origin regions and poorly organized termini. Collapse of replication fork 1 stalls replichore II while fork movement through replichore I continues unabated. E. Rapid cell growth triggers reinitiation at both oriCs. If forks 3 and 5 overtake stalled fork 1, double-strand ends expose red/blue strands to RecBCD degradation. The residual elements of forks 3 and 5 (black/blue strands) can restart one fork while forks 4 and 6 proceed to the terminus. In a RecA mutant, rescue of DNA ends by strand invasion is impaired, which leads to degradation of the partial chromosomal replicas with red/blue strands. F. The outcome is two genome equivalents of DNA having both an original template (black) and one blue strand derived from the second initiation.