
<ArticleTitle>Transcription-independent <prot>RNA polymerase II</prot> dephosphorylation by the <prot><prot>FCP1</prot> carboxy-terminal domain phosphatase</prot> in Xenopus laevis early embryos.</ArticleTitle>
<AbstractText>The phosphorylation of the <prot>RNA polymerase II</prot>(<prot>RNAP II</prot>) carboxy-terminal domain (CTD) plays a key role in mRNA metabolism. The relative ratio of hyperphosphorylated <prot>RNAP II</prot> to hypophosphorylated <prot>RNAP II</prot> is determined by a dynamic equilibrium between <prot>CTD kinases</prot> and <prot>CTD phosphatase</prot>(s). The CTD is heavily phosphorylated in meiotic Xenopus laevis oocytes. In this report we show that the CTD undergoes fast and massive dephosphorylation upon fertilization. A cDNA was cloned and shown to code for a full-length <prot>xFCP1</prot>, the Xenopus orthologue of the <prot><prot>FCP1</prot> CTD phosphatases</prot> in humans and Saccharomyces cerevisiae. Two critical residues in the catalytic site were identified. <prot>CTD phosphatase</prot> activity was observed in extracts prepared from Xenopus eggs and cells and was shown to be entirely attributable to <prot>xFCP1</prot>. The CTD dephosphorylation triggered by fertilization was reproduced upon calcium activation of cytostatic factor-arrested egg extracts. Using immunodepleted extracts, we showed that this dephosphorylation is due to <prot>xFCP1</prot>. Although transcription does not occur at this stage, phosphorylation appears as a highly dynamic process involving the antagonist action of <prot>Xp42 mitogen-activated protein kinase</prot> and <prot><prot>FCP1</prot> phosphatase</prot>. This is the first report that free <prot>RNAP II</prot> is a substrate for <prot>FCP1</prot> in vivo, independent from a transcription cycle.</AbstractText>
