Abstract

TFIIF is the only general transcription factor that has been implicated in the preinitiation complex assembly, open complex formation, initiation and transcription elongation. In addition, TFIIF stimulates Fcp1, a central phosphatase needed for recycling of RNA polymerase II (Pol II) after transcription by dephosphorylation of the Pol II C-terminal domain (CTD). This thesis reports the X-ray structure of the small CTD phosphatase Scp1, which is homologous to the Fcp1 catalytic domain. The structure shows a core fold and an active center similar to phosphotransferases and –hydrolases that solely share a DXDX(V/T) signature motif with Fcp1/Scp1. It was further demonstrated that the first aspartate in the signature motif undergoes metalassisted phosphorylation during catalysis, resulting in a phosphoaspartate intermediate that was structurally mimicked with the inhibitor beryllofluoride. Specificity may result from CTD binding to a conserved hydrophobic pocket between the active site and an insertion domain that is unique to Fcp1/Scp1. Fcp1 specificity may additionally arise from phosphatase recruitment near the CTD via the Pol II subcomplex Rpb4/7, which is shown to be required for Fcp1 binding to the polymerase in vitro. Until now, the main impediment in the high resolution crystallographic studies of TFIIF in complex with Pol II and other members of transcription machinery was unavailability of soluble, stoichiometric TFIIF complex in sufficient amounts. This thesis reports on the development of the overexpression system in yeast and a purification protocol that enabled for the first time to isolate milligram amounts of a pure and soluble, 15-subunit (~0,7 MDa) stoichiometric Pol IITFIIF complex. Such complex together with the promoter DNA, RNA, TBP and TFIIB assembles in vitro into the yeast initially transcribing complex, which can now be studied structurally.