Résumé
The coding sequence of the proliferating cell nuclear antigen (PCNA) was characterized in the amphibian
Xenopus laevis. The deduced protein sequence shares an extensive homology (89%) with the mammalian PCNA coding sequences.
Xenopus PCNA is expressed beginning in early oogenesis and reaches a level of 3 × 10
7 transcripts per mature oocyte, whereas proliferative somatic cells contain 3 × 10
2 PCNA transcripts per cell. Most of the PCNA protein is expressed during late oogenesis and one single stage VI oocyte contains the amount of PCNA protein present in 4 × 10
5 somatic cells in culture. Thus most, if not all, of the PCNA required for early development is stored as a maternal gene product. Part of the mRNA stockpile is degraded during the cleavage stage and then new PCNA zygotic expression at the neurula stage maintains a constitutive value of 30 transcripts per cell until the tailbud stage. The maternal protein is maintained at a constant level during embryonic development at least until the swimming tadpole stage. The protein is localized in the nuclei at all stages of oogenesis and development that were examined.