Efficient plasmid DNA replication in Xenopus egg extracts does not depend on prior chromatin assembly

Abstract

Small plasmids replicate efficiently in unfertilized Xenopus eggs provided they are injected before rather than after activation of the cell cycle. Here we use Xenopus egg extracts to test the hypothesis that efficient replication results from chromatin assembly prior to activation giving preloaded plasmids a head start toward the formation of a replicating pseudonucleus (Sanchez, J. A., Marek, D., and Wangh, L. J. (1992) J. Cell Sci. 103, 907-918). As in ovum, plasmid DNA preincubated in unactivated egg cytoplasmcytostatic factor extracts) replicate more efficiently after extract activation than does the same DNA added to the same extract after activation. Unlike in ovum, however, plasmids that replicate efficiently in vitro do not assemble into chromatin during preincubation and become topologically knotted instead. But even DNA knotting does not explain subsequent efficient replication. Also, plasmids preassembled into chromatin in vitro do not replicate efficiently in activated egg cytoplasm unless first preincubated in a CSF extract. We conclude that unactivated eggs contain replication-enhancing activities that can act independently of plasmid chromatin assembly and DNA topology. These postulated 'preloading' factor(s) may be related to licensing factor, an activity that controls initiation of DNA replication in eukaryotic cells. The experimental conditions described here will permit characterization of preloading/licensing factor(s) in the context of a small plasmid substrate.