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p53 is a nuclear protein that acts like a tumor suppressor and is involved in regulation of cellular growth. In Xenopus, the p53 protein is highly expressed during oogenesis and is strictly cytoplasmic in the oocyte. We have analysed its participation in DNA replication and transcription during early development, using the egg and oocyte as model-systems. The injection of sperm nuclei into Xenopus eggs is followed by DNA replication and mitotic events. We show that the endogenous p53 enters the nuclei and moves through a series of discrete subnuclear loci whose distribution is S-phase speci®c. A speci®c peripheral nuclear localization of p53 is observed before entry into S-phase, followed by an internal localization which is strictly dependent on ongoing DNA synthesis. At no stage in the cell cycle, however, did we observe any co-localization with RPA or PCNA, which were used as initiation or elongation markers for DNA replication. We also show that injection into the nucleus of the oocyte of small amounts of either Xenopus or human p53 ± less than 10% of the cytoplasmic storage ± is su cient to block RNA polymerase IIdependent transcription from a coinjected TATA-boxcontaining reporter plasmid. Transcription is rescued by microinjection of the TATA-box binding protein (TBP), suggesting that nuclear exclusion of p53 during oogenesis may be necessary for transcription of maternal genes. These characteristics are discussed in relation to the regulation of nuclear activities during early embryogenesis.

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