Characterization and developmental expression of Xenopus proliferating cell nuclear antigen (PCNA).

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 X 10(7) transcripts per mature oocyte, whereas proliferative somatic cells contain 3 X 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 X 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.