Cdt1 interactions in the licensing process - A model for dynamic spatiotemporal control of licensing

Within each cell cycle, a cell must ensure that the processes of selection of replication origins ( licensing) and initiation of DNA replication are well coordinated to prevent reinitiation of DNA replication from the same DNA segment during the same cell cycle. This is achieved by restricting the licensing process to G1 phase when the prereplica tive complexes ( preRCs) are assembled onto the origin DNA, while DNA replication is initiated only during S phase when de novo preRC assembly is blocked. Cdt1 is an. important member of the preRC complex and its tight regulation through ubiquitin dependent proteolysis and binding to its inhibitor Geminin ensure that Cdt1 will only be present in G1 phase, preventing relicensing of replication origins. We have recently reported that Cdt1 associates with chromatin in a dynamic way and recruits its inhibitor Geminin onto chromatin in vivo. Here we discuss how these dynamic Cdt1 - chromatin interactions and the local recruitment of Geminin onto origins of replication by Cdt1 may provide a tight control of the licensing process in time and in space.