ORC binding, gene amplification, and the nature of metazoan replication origins
- Howard Hughes Medical Institute, Carnegie Institution of Washington, Baltimore, Maryland 21210 USA
This extract was created in the absence of an abstract.
Genomic DNA replication is a critical process shaped by strong evolutionary pressures. Rapid replication can give a viral or cellular genome an immediate advantage over its competitors as long as requisitely high accuracy is not sacrificed. Prokaryotes employ an extremely fast, accurate DNA replication mechanism utilizing sophisticated multiprotein machines (Kornberg and Baker 1992; Baker and Bell 1998). Genome duplication is controlled by the assembly and activation of bidirectional replication forks at specific replication origins, under the control of initiator proteins that bind origins sequence specifically. For example, Escherichia coli oriC binds multiple copies of the initiator protein, dnaA, at repeated motifs. In an ATP-dependent manner dnaA then locally unwinds the DNA, facilitates helicase loading, and organizes the assembly of polymerases, primases, and other components of the replication fork. Special proteins are required to assemble and maintain this unwieldy machinery. Thus, loading the ring-shaped six-protein dnaB helicase around the DNA requires dnaC, whereas proteins such as tau probably act as scaffolds, organizing replication fork components (including polymerases).
Despite the greater complexity of eukaryotic chromosomes, work carried out in Saccharomyces cerevisiae indicates that its genome replicates via a strongly similar mechanism (Leatherwood 1998). Each yeast chromosome contains multiple discrete replication origins where prereplication complexes are assembled during late M and G1 phases. Fully formed, bidirectional replication forks are then activated during S phase. Replication origins can be readily identified as autonomously replicating sequence (ARS) elements that support the propagation of extrachromosomal plasmids. Budding yeast origins are ∼100 bp in size and share an A/T-rich ARS consensus sequence (ACS) that is essential for origin function. Six proteins that form the origin recognition complex (ORC) bind to the ACS in an ATP-dependent manner throughout the cell cycle (Bell and Stillman 1992). Beginning in late mitosis, Cdc6p interacts with origin-bound ORC to bring the …
