Over-replication in E. coli recG cells with an ectopic termination area. RecG ectopic ter

Chromosome duplication initiates via the assembly of replication forks at defined origins. Forks proceed in opposite directions until they fuse with a converging fork. Recent work highlights that fork fusions are highly choreographed both in pro- and eukaryotic cells. In Escherichia coli the circular chromosome is replicated from a single origin (oriC), dictating that only a single fork fusion takes place. This occurs in a specialised area opposite oriC that forms a fork trap via ter/Tus complexes, allowing forks to enter but not leave. A surprising number of proteins appear to be involved in the processing of fork fusion intermediates. Here we further define the events associated with fork fusions and the role of RecG helicase in replication termination. Our data support the idea that events associated with head-on fork fusions cause ectopic replication initiation in ΔrecG cells. This over-replication occurs in a significant proportion of cells and is dramatically elevated when one fork is trapped for some time before the converging fork arrives. We suggest that the termination area evolved to contain such over-replication. In addition, our data suggest that the stable arrest of replication forks at ter/Tus complexes is an important feature to prevent problems as replication terminates.