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J. Cell Biol.,
Volume 140, Number 2, January 26, 1998 271-281
Biology Department, University of California, San Diego, CA 92093-0347
Before initiation of DNA replication, origin
recognition complex (ORC) proteins, cdc6, and minichromosome maintenance (MCM) proteins bind to
chromatin sequentially and form preinitiation complexes. Using Xenopus laevis egg extracts, we find that
after the formation of these complexes and before initiation of DNA replication, cdc6 is rapidly removed from
chromatin, possibly degraded by a cdk2-activated,
ubiquitin-dependent proteolytic pathway. If this displacement is inhibited, DNA replication fails to initiate.
We also find that after assembly of MCM proteins into
preinitiation complexes, removal of the ORC from
DNA does not block the subsequent initiation of replication. Importantly, under conditions in which both
ORC and cdc6 protein are absent from preinitiation
complexes, DNA replication is still dependent on cdk2
activity. Therefore, the final steps in the process leading to initiation of DNA replication during S phase of the
cell cycle are independent of ORC and cdc6 proteins,
but dependent on cdk2 activity.
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