Chromosomal over-replication in Escherichia coli recG cells is triggered by replication fork fusion and amplified if replichore symmetry is disturbed.

Sarah L Midgley-Smith,Robert G Lloyd,Toni Taylor,Nicole M Forrester,Christian J Rudolph,Amy L Upton,Juachi U Dimude

doi:10.1093/nar/gky566

Sarah L Midgley-Smith, Robert G Lloyd + Show 5 more

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https://doi.org/10.1093/nar/gky566

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Abstract

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. The circular Escherichia coli chromosome is replicated from a single origin (oriC), and a single fork fusion takes place in a specialised termination area opposite oriC that establishes a fork trap mediated by Tus protein bound at ter sequences that allows forks to enter but not leave. Here we further define the molecular details of fork fusions and the role of RecG helicase in replication termination. Our data support the idea that fork fusions have the potential to trigger local re-replication of the already replicated DNA. In ΔrecG cells this potential is realised in a substantial fraction of cells and is dramatically elevated when one fork is trapped for some time before the converging fork arrives. They also support the idea that the termination area evolved to contain such over-replication and we propose that the stable arrest of replication forks at ter/Tus complexes is an important feature that limits the likelihood of problems arising as replication terminates.

Highlights

Every time a cell divides, its DNA content has to be replicated and transmitted to its daughter cells with high fidelity [1]
Viability is expected to be retained in this case, because even if replication forks coming from oriC would be blocked, synthesis coming from oriZ would complete chromosome duplication
In this study we have further defined the molecular role that RecG plays in preventing over-replication in the termination area of the chromosome and the molecular mechanisms involved in triggering this over-replication

Summary

Introduction

Every time a cell divides, its DNA content has to be replicated and transmitted to its daughter cells with high fidelity [1]. Replication is completed when converging forks fuse within a specialized termination region flanked by polar ter sequences (terA–J) that are bound by the Tus terminator protein (Figure 1A) [3,4]. This region contains specialized genetic elements such as the dif site that facilitates resolution of any chromosome dimers and KOPS sequences which guide proteins facilitating the segregation of duplicated DNA to daughter cells [5,6]. The chromosome is divided into two approximately equal halves called replichores, each replicated by a single replication fork complex [9]

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