Termination of Eukaryotic Replication Forks

Abstract

Termination of DNA replication forks takes place when two replication forks coming from neighbouring origins meet each other usually in the midpoint of the replicon. At this stage, the remaining fragments of DNA have to be unwound, all remaining DNA replicated and newly synthesised strands ligated to produce continuous sister chromatids. Finally, the replication machinery has to be taken off, chromatin re-assembled, and entwisted sister chromatids resolved topologically.Over the last few decades, we have learned a lot about the assembly of the helicase and replisome and the initiation stage of DNA replication. We also know much more about the ability of forks to cope with replication stress. However, only within recent years we have gained the first glimpse of the mechanism of replication fork termination. In this chapter I will summarise the recent findings on replication termination, weigh this against the past literature and discuss relevant consequences and views for the future.

Too Much of a Good Thing: How Ectopic DNA Replication Affects Bacterial Replication Dynamics

“…This does not mean that the fusion of forks is unproblematic in eukaryotic cells. On the contrary, recent work has highlighted that replisome disassembly is highly choreographed and that multiple accessory proteins, such as the helicases Rrm3 and Pif1, are necessary for bringing replication to an accurate conclusion (Steinacher et al, 2012;Maric et al, 2014;Moreno et al, 2014;Dewar et al, 2015;Moreno and Gambus, 2015;Dewar and Walter, 2017;Gambus, 2017;Deegan et al, 2019), highlighting that we have only just started to understand the mechanisms and regulation of fork fusion events both in bacteria and eukaryotes.…”

“…Similarly, no specific termination-related pause sites have been identified in eukaryotes and archaea, even though multiple replication origins per chromosome result in a much higher number of fork fusions. It appears that replication effectively terminates at random locations between origins (Duggin et al, 2011;Hawkins et al, 2013;Samson et al, 2013;Gambus, 2017).…”

CMG helicase disassembly is controlled by replication fork DNA, replisome components and a ubiquitin threshold

“…This is important, because displacement of CMG from DNA blocks fork progression irreversibly ( Labib et al, 2000 ), since CMG cannot be reassembled at a replication fork during S-phase. Nonetheless, the convergence of two replication forks leads very quickly to CMG disassembly and replisome dissolution ( Dewar and Walter, 2017 ; Gambus, 2017 ), which is likely to represent the final stage of DNA replication termination. Consistent with this view, CMG unloading occurs after the formation of a fully ligated DNA product during plasmid replication in Xenopus laevis egg extracts ( Dewar et al, 2015 ).…”

Termination ofDNAReplication in Prokaryotes

“…In E. coli and B. subtilis, the location of fork fusion is restricted to the centre of the terminus region by several replication pause sites (explained further below). In contrast, in diverse eukaryotes and archaea that have multiple replication origins per chromosome, fork fusion does not appear to be controlled by specific pause sites in many cases and instead occurs at random sites within fork fusion zones between origins (Duggin et al, 2011;Gambus, 2017;Hawkins et al, 2013;Samson et al, 2013).…”