The DNA replication licensing system.

The Xenopus cell free system has proved a good model system to study in vitro DNA replication and the mechanism preventing rereplication in a single cell cycle. Studies using this system resulted in the development of a model postulating the existence of a replication licensing factor (RLF), which binds to the chromatin before the G1-S transition of the cell cycle and is displaced during replication. The nuclear envelope prevents rebinding of RLF and hence relicensing. Nuclear envelope breakdown at mitosis is required to allow another round of replication. Protein kinase inhibitors block licensing factor activity and arrest Xenopus extracts in a G2 like state. These kinase inhibitors have allowed the development of an in vitro assay leading to the biochemical purification of RLF components. RLF can be separated into RLF-B and RLF-M, the latter consisting of several members of the MCM/P1 class of replication proteins. In Xenopus as well as in many other eukaryotes, the binding of MCM/P1 proteins to chromatin before S phase is essential for replication to occur. The proteins are then displaced as replication proceeds. These changes in subnuclear distribution are reflected by changes in the phosphorylation status. MCM/P1 proteins do not bind to the DNA on their own but need RLF-B to be loaded onto the chromatin. Their cycling behaviour is reminiscent of the existence of a prereplicative complex at the origins of replication in yeast, suggesting that the licensing mechanism is ubiquitous in eukaryotes.