Chromosome replication in cell-free systems from Xenopus eggs.

Cell-free systems from eggs of the frog Xenopus laevis are able to perform most of the acts of eukaryotic chromosome replication in vitro. This now includes the crucial regulatory step of initiation, which had only been achieved for viral systems previously. Purified DNA or nuclei are able to initiate and complete semi-conservation replication in egg extracts in vitro (Blow & Laskey, Cell 47, 557-587 (1986). Replication does not require specialized DNA sequences either in vitro or in microinjected eggs, but in both systems large templates replicate more efficiently than small templates. In some cases replication can re-initiate, excluding the possibility that replication is primed by preexisting primers in the template preparations. When nuclei are replicated in vitro, only one round of replication is observed in a single incubation resembling the single round of replication observed for purified DNA after micro-injection. The mechanism that prevents re-initiation of replication within a single cell cycle is discussed and certain models are eliminated. Nucleosome assembly from histones and DNA has also been studied in cell-free systems from Xenopus eggs. Fractionation has led to the identification of two acidic proteins called nucleoplasmin and N1, which bind histones and transfer them to DNA. The sequences of both proteins have been determined by cDNA cloning and sequencing. Both proteins are found as complexes with histones in eggs.