Mechanisms of DNA Replication Termination

Genome duplication is carried out by pairs of replication forks that assemble at origins of replication and then move in opposite directions. DNA replication ends when converging replication forks meet. During this process, which is known as replication termination, DNA synthesis is completed, the replication machinery is disassembled and daughter molecules are resolved.

The Process of Fork Convergence

Termination of DNA replication occurs when two replication forks meet on the same stretch of DNA, during which the following events occur, although not necessarily in this order: forks converge until all intervening DNA is unwound; any remaining gaps are filled and ligated; catenanes are removed; and replication proteins are unloaded. In this Review, we outline the steps that are likely to be common to replication termination in most organisms, namely, fork convergence, synthesis completion, replisome disassembly and decatenation.

Comparative Mechanisms in Different Organisms

Replication termination can be a problematic process. In eukaryotes, most termination sites are determined stochastically by the location of replication initiation sites. In bacteria, termination generally occurs at a specific locus. Regarding specialized cases, termination of simian virus 40 (SV40) replication involves the stalling of converging forks, and bacterial termination is prone to inducing re-replication. By contrast, fork stalling or re-replication have not been observed during unperturbed termination in eukaryotes.

Topological Stress and Synthesis Completion

Topological stress accumulates between converging forks and is relieved by the generation of pre-catenanes, which are removed by type II topoisomerases. During bacterial and SV40 termination, type II topoisomerases are required for fork convergence, but in eukaryotes they are dispensable for this purpose. After forks converge, any remaining catenanes are removed by a type II topoisomerase. Furthermore, in eukaryotes, gaps are readily filled by the extension of the leading strands, but in bacteria and SV40 this process is less well-defined.

Replisome Disassembly and Genome Instability

In eukaryotes, a dedicated replisome removal pathway has recently been identified, which operates late during termination, after the DNA is fully replicated. It is unclear whether any comparable pathway exists in bacteria. We focus on recent advances in eukaryotic replication termination, in particular, the recently discovered E3 ubiquitin ligases that control replisome disassembly in yeast and higher eukaryotes, and how their activity is regulated to avoid genome instability.

Comparison Summary of Termination Characteristics

• Eukaryotes: Most termination sites are determined stochastically; type II topoisomerases are dispensable for fork convergence; gaps are readily filled by leading strand extension; dedicated replisome removal pathway via E3 ubiquitin ligases exists.
• Bacteria (E. coli): Termination generally occurs at a specific locus; type II topoisomerases are required for fork convergence; prone to inducing re-replication.
• Simian Virus 40 (SV40): Involves the stalling of converging forks; type II topoisomerases are required for fork convergence; gap filling process is less well-defined.