Qualifying for the license to replicate

Abstract

Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco,California 94143-0448A single replication fork would take more than a year to replicate the genome of Xenopus. Bydividing the task among many thousands of replicons, each replicated by forks emanating fromindividual origins, replication is instead completed in as little as 30 min. While this efficientstrategy has been adopted by all eukaryotes, it introduces a complication. To maintain theintegrity of the genome, multiple replicons must now be coordinated so that all sequences arereplicated exactly once per cell cycle. Because different replicons are often replicated atdifferent times during S phase, replicated regions must be distinguishable from unreplicatedregions to avoid problems of rereplication. We suggest that this distinction is based upon afundamental feature of replication initiation.Two things happen at origins of replication: proteins are recruited to origins to assemblemultiprotein replication machines (Figure 1, left), and these assemblies are triggered to initiatereplication forks (Figure 1, right). Replication components accompany the departing forks,leaving behind a spent origin. Consequently, reinitiation should require assembly of newcomponents at the origin. If this assembly is restricted to one part of the cell cycle and theinitiation of forks to another, then origin firing would occur only once per cell cycle (Figure1).The transition between replication-competent and replication-incompetent phases of the cellcycle has been explored in a series of early and influential cell fusion experiments (Rao andJohnson, 1970). Upon fusion with an S phase cell, nuclei from G1 cells, but not from G2 cells,replicate their DNA. Thus, even when present in cytoplasm capable of supporting S phase, theG2 nucleus is incompetent to replicate. Since G2 nuclei are converted into G1 nuclei by thepassage through mitosis, mitosis must provide replication competence to the G2 nucleus. Inthe last several years, in vitro experiments using Xenopus egg extracts as well as geneticexperiments using fission yeast have given rise to two rather different models for the basis ofthis mitotic transition. We outline each of these areas of research below and evaluate featuresof each model in an attempt to bring us closer to a unified understanding of these events.