DEVELOPMENTAL-CHANGES IN RNA-POLYMERASE-II IN BOVINE OOCYTES, EARLY EMBRYOS, AND EFFECT OF ALPHA-AMANITIN ON EMBRYO DEVELOPMENT

Development of mammalian early embryos relies on stored maternal messe nger RNAs (mRNAs) that have been synthesized during oogenesis until em bryonic genome activation. Although embryonic genome activation in bov ine embryos has been proposed to start at the late 4-cell stage, recen t evidences suggest that embryonic genome activation starts earlier th an the 4-cell stage, and molecular details of this event are not known . RNA polymerase II in eukaryotes is responsible for transcription of mRNA and most of the small nuclear RNAs. The unphosphorylated form of RNA polymerase II (IIA) has been shown to function in transcriptional initiation, and the hyperphosphorylated form (IIO) functions in transl ational elongation and mRNA splicing. In this study, we examined the c hanges in the amount of RNA polymerase IIA by immunoblotting in immatu re oocytes; mature oocytes; and 2-, 4- and 8-cell bovine embryos. We a lso examined the levels of IIO and the multiple intermediately phospho rylated form in the same oocytes and embryos. The IIA reached the high est level at the 2-cell stage and decreased gradually at the 4- and 8- cell stages, and IIO was at very low levels in mature oocytes and 2-ce ll stage embryos and was not detectable at later stages. The multiple intermediately phosphorylated form was present at the highest level in mature oocytes and was detectable at the other stages. We demonstrate that RNA polymerase IIA, which is responsible for initiation of trans cription, is present in oocytes and preimplantation embryos and reache s the highest levels in the 2-cell stage embryos. Inhibition of RNA po lymerase II-dependent transcription during any of the first four embry onic cell cycles has detrimental effects on progression of embryonic d evelop ment beyond the 16-cell stage, indicating the importance of ear ly transcripts for continuation of development. The results indicate t hat expression of all the genes whose transcription is inhibited by al pha-amanitin is essential for embryo development, Mol. Reprod. Dev. 51 :381-389, 1998. (C) 1998 Wiley-Liss, Inc.