Transcription-independent RNA polymerase II dephosphorylation by the FCP1 carboxy-terminal domain phosphatase in Xenopus laevis early embryos

The phosphorylation of the RNA polymerase II (RNAP II) carboxy-terminal dom ain (CTD) plays a key role in mRNA metabolism. The relative ratio of hyperp hosphorylated RNAP II to hypophosphorylated RNAP II is determined by a dyna mic equilibrium between CTD kinases and CTD phosphatase(s). The CTD is heav ily phosphorylated in meiotic Xenopus laevis oocytes. In this report we sho w that the CTD undergoes fast and massive dephosphorylation upon fertilizat ion. A cDNA was cloned and shown to code for a fall-length xFCP1, the Xenop us orthologue of the FCP1 CTD phosphatases in humans and Saccharomyces cere visiae. Two critical residues in the catalytic site were identified. CTD ph osphatase activity was observed in extracts prepared from Xenopus eggs and cells and was shown to be entirely attributable to xFCP1. The CTD dephospho rylation 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 xFCP1. Although transcripti on does not occur at this stage, phosphorylation appears as a highly dynami c process involving the antagonist action of Xp42 mitogen-activated protein kinase and FCP1 phosphatase. This is the first report that free RNAP II is a substrate for FCP1 in vivo, independent from a transcription cycle.