Inhibition of DNA replication in cell-free extracts of Xenopus laevis eggs by extracts of Xenopus laevis oocytes.

Cell-free extracts of Xenopus laevis eggs support replication of Xenopus sperm nuclei and purified double-stranded plasmid DNA templates. In contrast, cell-free extracts of Xenopus oocytes do not support replication of these templates. In this study, we show that extracts prepared from Xenopus oocytes, when added to extracts of Xenopus eggs, inhibited replication of Xenopus sperm nuclei and double-stranded plasmid DNA templates. This inhibition did not result from degradation of template DNA by oocyte extracts, nor from inhibition of chain elongation. Nuclei formed in egg extracts in the presence of oocyte extracts were much smaller compared with nuclei formed in egg extracts. Extracts prepared from Xenopus germinal vesicles (oocyte nuclei) also inhibited replication of Xenopus sperm nuclei, but not synthesis of the complementary strand of single-stranded DNA templates in egg extracts. In contrast to extracts prepared from immature oocytes, extracts prepared from Xenopus oocytes matured in vitro effect much less, if any, inhibition of DNA replication. Taken together, these results suggest that molecules that negatively regulate chromosomal DNA replication may be present in Xenopus immature oocytes. The level of these factors may decrease during oocyte maturation, concurrent with the breakdown of oocyte germinal vesicles and manifestation of the capacity of matured oocytes to initiate DNA replication. To our knowledge, this is the first biochemical identification of negative factors that may be involved in the regulation of eukaryotic chromosomal DNA replication.