Résumé
ORIGINS of DNA replication in
Saccharomyces cerevisiae
are bound by two protein complexes during the cell cycle
1,2
. Post-replicative complexes closely resemble those generated in vitro by purified origin recognition complex (ORC)
1,3–5
, which is essential for DNA replication
in vivo
6–11
. Pre-replicative complexes (pre-RCs) are characterized by an extended region of nuelease protection overlapping the ORC footprint
1
. We show here that the Cdc6 protein (Cdc6p), which is necessary for origin firing
in vivo
12–16
, is essential for the establishment and maintenance of pre-RCs, suggesting that it is a component of these complexes. Without Cdc6p, G1 origins closely resemble post-replicative origins, providing evidence that ORC is also a component of pre-RCs. These results suggest that pre-RCs play an essential role in initiating DNA replication and support a two-step mechanism for the assembly of functional initiation complexes.