Cytoplasmic control of nuclear DNA synthesis during early development of Xenopus laevis: a cell-free assay.

Nuclei isolated from nondividing cells were induced to synthesize DNA by incubation with cytoplasm from early embryos of Xenopus laevis. Numerous replication eyes were formed in the nuclear DNA molecules, and high levels of [3-H]dTTP were incorporated. With this assay a protein(s) which appears to initiate DNA synthesis was found at high levels in the cytoplasm of eggs, blastulae, or gastrulae, but only at low levels in the cytoplasm of oocytes, hatched embryos, or adult tissues.