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

Termination of DNA replication at Tus-terbarriers results in under-replication of template DNA

“…Our results pose the obvious question as to the evolutionary advantage of Tus-ter for the E. Indeed, a fork trap introduces a significant level of constraint to chromosome duplication: if one fork is stalled at an obstacle before it has progressed through the first five ter sites it encounters and cannot be reactivated, the second fork will be unable to rescue this blocked fork because it will be blocked by Tus-ter complexes. If the stalled fork cannot be reactivated, the cells are in danger of dying (3,8), a problem that will not arise in the same way in bacterial chromosomes without a fork trap. We were recently able to recreate this scenario in vivo, by moving the origin from its original location into either the right-hand or left-hand replichore (41,42).…”

“…We were recently able to recreate this scenario in vivo , by moving the origin from its original location into either the right-hand or left-hand replichore (41, 42). In these cells, replisomes coming from ectopic origins have to duplicate a significant proportion of the chromosome in an orientation opposite to normal, which results in replication-transcription conflicts that will delay the progression of one fork, whilst the second fork is stalled at Tus- ter complexes (8,41,42). The resulting cells show a significant growth defect, indicating that this scenario can cause serious issues for the cell.…”

“…It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for this this version posted February 25, 2021. ; https://doi.org/10.1101/2021.02.25.432933 doi: bioRxiv preprint RecG (17) and 3'-exonucleases (22,23). These enzymes are thought to be involved in the processing of intermediates that form as a result of fork fusion events and can lead to replication restart if the intermediates persist for longer than normal (8,23). This model is supported by the fact that over-replication is either eradicated or significantly reduced in cells lacking either RecG or 3'-exonucleases following linearisation of the chromosome in the termination area, which will prevent replisomes from fusing (23)(24)(25).…”

“…Like the majority of bacteria, E. coli has a single circular chromosome which is replicated bidirectionally from a single replication origin ( oriC ). The single origin dictates that each chromosomal half or ‘replichore ‘is duplicated by a single replisome and the number of replication fork fusions is restricted to exactly one, which takes place midway around the chromosome (8) (Figure 1A). The location of termination is constrained by a specialised replication fork trap formed by a series of polar blocks which allow replisomes to enter but not leave the termination region.…”

Can a nick promote partial genome re-replication?

“…A new replisome can then assemble at this primer to complete replication. In Escherichia coli , each of the two replication forks from the single chromosomal replication origin ( oriC ) must travel over 2Mb and are terminated in a zone ~180° from the origin; consequently replication restart is the only way to complete replication after fork collapse [1, 3]. In eukaryotes, the genome is replicated from multiple origins and individual replication forks travel much shorter distances than in E.coli .…”