Xenopus Cdc7 function is dependent on licensing but not on XORC, XCdc6, orCDK activity and is required for XCdc45 loading

The assembly and disassembly of protein complexes at replication origins pl ay a crucial role in the regulation of chromosomal DNA replication. The seq uential binding of the origin recognition complex (ORC), Cdc6, and the mini chromosome maintenance (MCM/P1) proteins produces a licensed replication or igin. Before the initiation of replication can occur, each licensed origin must be acted upon by S phase-inducing CDKs and the Cdc7 protein kinase. In the present report we describe the role of Xenopus Cdc7 (XCdc7) in DNA rep lication using cell-free extracts of Xenopus eggs. We show that XCdc7 binds to chromatin during G(1) and S phase. XCdc7 associates with chromatin only once origins have been licensed, but this association does not require the continued presence of XORC or XCdc6 once they have fulfilled their essenti al role in licensing. Moreover, XCdc7 is required for the subsequent CDK-de pendent loading of XCdc45 but is not required for the destabilization of or igins that occurs once licensing is complete. Finally, we show that CDK act ivity is not necessary for XCdc7 to associate with chromatin, induce MCM/P1 phosphorylation, or perform its essential replicative function. From these results we suggest a simple model for the assembly of functional initiatio n complexes in the Xenopus system.