FORMATION OF A PREINITIATION COMPLEX BY S-PHASE CYCLIN CDK-DEPENDENT LOADING OF CDC45P ONTO CHROMATIN

Cdc45p, a protein essential for initiation of DNA replication, associa tes with chromatin after ''start'' in late G(1) and during the S phase of the cell cycle. Binding of Cdc45p to chromatin depends on Clb-Cdc2 8 kinase activity as well as functional Cdc6p and Mcm2p, which suggest s that Cdc45p associates with the prereplication complex after activat ion of S-phase cyclin-dependent kinases (CDKs). As indicated by the ti ming and the CDK dependence, binding of Cdc45p to chromatin is crucial for commitment to initiation of DNA replication, During S phase, Cdc4 5p physically interacts with minichromosome maintenance (MCM) proteins on chromatin; however, dissociation of Cdc45p from chromatin is slowe r than that of MCMs, which indicates that the proteins are released by different mechanisms.