Despite the large amount of work dedicated to its study, DNA replication is still not fully understood.
Recently a group from Oxford university has demonstrated that in E. coli a replication fork contains 3 polymerases and not 2 as it was expected since a priori there is only one needed for each of the 2 DNA strands.
The role of this supplementary polymerase was unknown.
In a collaborative work between a group from the CGM in Gif and a group from the Laboratoire de physique statistique de l’ENS and the ENS biology department, researchers used a single molecule detection technique to monitor in real time the protein composition of the replisome in a living E. coli. The authors could directly observe that at least one polymerase from the replisome is regularly exchanged, typically every second, with one diffusing in the cellular medium.
By observing in real time the amount of Single Strand Binding proteins at the replication fork, the authors noticed that this polymerase exchange is strongly correlated with the Okazaki fragment synthesis. They could then deduce that the exchanged polymerase is involved in the lagging strand synthesis.
This observation led them to propose that the third polymerase is likely present at the fork to replace the lagging strand one in cases where the capture of a new polymerase from the cellular medium is delayed. This "replacement" polymerase would be for DNA replication like a spare wheel for your car.
Reference G. Lia, B. Michel, J.F. Allemand, Polymerase Exchange during Okazaki Fragment Synthesis observed in Living Cells, Science, 335, 328 (2012), DOI : 10.1126/science.1210400 ;