Eukaryotic DNA Replication Origins Reconciling Disparate Data

A new insight is revealed by Anglana et al. 128 kb of the single, primary (high frequency) oriGNA13. mary origin represses initiation at the secondary origins. Moreover, increasing the nucleotide pool by addition of DNA precursors to the culture medium reduces the frequency of initiation at secondary origins, whereas reducing the nucleotide pool by addition of hydroxyurea (a specific inhibitor of ribonucleotide reductase) increases Eukaryotic DNA Replication Origins: the frequency of initiation at secondary origins. This implies that synchronization of cells at their G1/S boundary by reducing nucleotide pools would favor the appearance of initiation zones, a caveat that may account for some of the data in the literature. It also implies that the In this issue of Cell, Anglana and coworkers provide frequency of initiation sites in mammals is similar to the new insight into the nature of mammalian replication frequency in yeast and in frog eggs (1/20 to 1/30 kb). origins that helps to reconcile the divergent views that How might nucleotide pools affect initiation site selec-emerged over the past decade. Taken together with tion? Four possibilities come to mind. First, the ability other studies, we can see how replication origins have of ORC to assemble pre-RCs at specific DNA sites may evolved in response to the demands of animal devel-depend on dNTPs, as it does on ATP (Bell and Dutta, opment. 2002). Second, previous studies have shown that initiation at one origin will prevent initiation at an identical About 13 years ago, three papers appeared in Cell, each origin many kb away, presumably because replication claiming to have mapped initiation sites for DNA replica-forks passing through replication origins prevent either tion in Chinese hamster cells within the intergenic region assembly or activation of pre-RCS. Reducing the con-downstream of the DHFR gene. Remarkably, two of centration of dNTPs reduces the rate of DNA synthesis, these studies concluded that initiation events originated thus allowing more time for initiation at secondary ori-from specific genomic sites (Burhans et al., 1990; Han-gins. Third, inhibiting DNA synthesis can result in exten-deli et al., 1989), while the third concluded that initiation sive DNA unwinding in the absence of concomitant DNA events were distributed uniformly throughout the in-synthesis. In that event, DNA polymerase ␣:DNA pri-tergenic region (Vaughn et al., 1990). But despite the mase, the enzyme that initiates DNA synthesis de novo, development and application of novel origin mapping may begin on single-stranded DNA some …