The Escherichia coli chromosome moves to the replisome

Abstract The replisome, a large multi-subunit assembly, carries out the process of chromosome replication, connecting the unwrapping of the parental DNA with the creation of new daughter strands. In Escherichia coli, it is debated whether the two replisomes move independently along the two chromosome arms during replication or if they remain in close proximity, with the DNA being pulled toward the replisomes as replication progresses. Here, we use fluorescence microscopy to determine the location and diffusivity of the replisome and various chromosomal loci throughout the cell cycle of the model organism E. coli. We find that (i) the two replisomes are confined to a region of 250 nm and 120 nm along the cell long and short axis respectively, and the chromosomal loci move through this region sequentially based on distance from the origin of replication. (ii) When a locus is being replicated, its diffusivity slows down. (iii) There is no indication that replication initiation occurs close to the cell membrane as has been proposed in a few previous studies. In conclusion, our data supports a model with DNA moving towards stationary replisomes at replication.