Putting It All Together: Building a Prereplicative Complex

Carol S. Newlon essential for DNA replication and is required for the assembly and maintenance of the pre-RC, are excellent Department of Microbiology and Molecular Genetics candidates for components of the pre-RC. Other candidates include products of genes known to be required UMDNJ–New Jersey Medical School Newark, New Jersey 07103 for the initiation of DNA replication, including the mini-chromosome maintenance (MCM) proteins, Cdc45p, and the Cdc7p protein kinase and its regulatory subunit Dbf4p. The MCM proteins were first identified in a screen The initiation of DNA replication in eukaryotic chromo-for mutations that caused defects in the maintenance somes is best understood in the budding yeast Saccha-of plasmids in yeast, i.e., mutants in which plasmids romyces cerevisiae. In this organism, DNA replication were unstable (reviewed by Newlon, 1996). Six genes initiates at replication origins spaced at intervals of identified in this screen, MCM2–MCM7, encode a family approximately 40 kb whose activity depends upon cis-of related proteins that are conserved among eukaryotes acting replicator sequences called autonomously repli-(reviewed by Dutta and Bell, 1997). Members of this cating sequence (ARS) elements. Replicator activity family share a 240–amino acid domain that contains depends upon a short DNA sequence, the ARS consen-the motifs characteristic of DNA-dependent ATPases. sus sequence, which acts as the core of the recognition Adding to the interest in Cdc6p and the MCM family of sequence for the origin recognition complex (ORC) (re-proteins as components of the pre-RC are experiments viewed by Newlon, 1996). ORC, a six-polypeptide com-in Xenopus egg extracts which demonstrated that asso-plex, is essential for the initiation of DNA replication and ciation of MCM proteins with sperm chromatin, in a appears to be the yeast replication initiator protein, the reaction that requires Xenopus ORC and Cdc6p, is nec-analog of E. coli dnaA and SV40 T antigen (reviewed essary for the initiation of DNA replication (reviewed by by Dutta and Bell, 1997). The binding of ORC to ARS Dutta and Bell, 1997). elements depends upon ATP, and it has recently been Several recent papers report important progress in shown that ORC is an ATPase whose activity is modu-teasing apart the pre-RC and provide tantalizing sug-lated by sequence-dependent binding to DNA (Klemm gestions for the roles of its components (Aparicio et al., et al., 1997). cis-acting replicators have not been identi-1997; Donovan et al., 1997; Liang and Stillman, 1997; fied in metazoans, where replication appears to initiate and Tanaka et al., …