Incomplete RNA polymerase II phosphorylation in Xenopus laevis early embryos

Phosphorylation of RNA polymerase II largest subunit on its C-terminal doma in (CTD) heptapeptide repeats has been shown to play a key role in the regu lation of mRNA synthesis and processing. In many higher metazoans, early em bryos do not synthesise mRNAs during the first cell cycles following fertil isation. Transcription resumes and becomes an absolute requirement for deve lopment after several cell cycles characteristic of each species. Therefore , CTD phosphorylation has been investigated during early development of the African clawed-frog Xenopus laevis. Fertilisation is shown to trigger an a brupt dephosphorylation of the CTD. Phosphorylation of the CTD resumes conc urrently with the mid-blastula transition (MBT). Both are advanced with pol yspermy and increased temperatures; they do not occur when replication is i mpaired with aphidicolin. In Xenopus laevis somatic cells, a set of monoclo nal antibodies defined distinct phosphoepitopes on the CTD. Two of them wer e absent before the MBT indicating that the CTD lacks the phosphorylation a t the serine-2 position of the heptapeptide. The possible contribution of R NA polymerase II phosphorylation to the developmental-regulation of materna l mRNA processing in embryos is discussed.