Epigenetic regulation of eukaryotic DNA replication origins

In higher eukaryotes, DNA replication initiates at multiple sites distributed along the chromosome, called replication origins. The location and timing of these initiation events are developmentally and epigenetically regulated. In the early Xenopus embryos replication origins are more abundant than in somatic cells, or at later stages of development. This particular organization of the replicon ensures rapid and accurate genome duplication during early embryonic cell divisions. Close to the mid-blastula transition (MBT), the density of replication origins declines, initiation of DNA replication becomes restricted to specific sites and correlates with cell cycle lengthening. Intriguingly, in nuclear transfer experiments exposure of somatic nuclei to egg cytoplasm is sufficient to convert the somatic origin density to the embryonic one, suggesting that the replication origin assembly in somatic cells is restrained by epigenetic factors modifiable by the egg cytoplasm. We performed extensive biochemical fraction of egg cytoplasm to identify factors that regulate replication origin density in embryonic and somatic nuclei. This effort led to the isolation of SSRP1 as major factor that stimulates somatic nuclei replication in egg extract. We showed that exposure of somatic nuclei to SSRP1 removes epigenetic determinants that restrain replication origin assembly and restores replication origin density typical of embryonic nuclei on somatic chromatin. Strikingly, SSRP1-mediated regulation of replication origin assembly has a dramatic impact on cell cycle progression and early embryonic development. These results and their implications will be described in details in the context of more general regulation of DNA replication in vertebrate organisms.