A FUNCTIONAL-ANALYSIS OF P53 DURING EARLY DEVELOPMENT OF XENOPUS-LAEVIS

p53 is a nuclear protein that acts like a tumor suppressor and is invo lved in regulation of cellular growth, In Xenopus, the p53 protein is highly expressed during oogenesis and is strictly cytoplasmic in the o ocyte, We have analysed its participation in DNA replication and trans cription during early development, using the egg and oocyte as model-s ystems. The injection of sperm nuclei into Xenopus eggs is followed by DNA replication and mitotic events, We show that the endogenous p53 e nters the nuclei and moves through a series of discrete subnuclear loc i whose distribution is S-phase specific, A specific peripheral nuclea r 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 el ongation 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 sufficient to blo ck RNA polymerase II-dependent transcription from a coinjected TATA-bo x-containing reporter plasmid. Transcription is rescued by microinject ion of the TATA-box binding protein (TBP), suggesting that nuclear exc lusion of p53 during oogenesis may be necessary for transcription of m aternal genes, These characteristics are discussed in relation to the regulation of nuclear activities during early embryogenesis.