Protein kinase essential for cell proliferation, where it is required for completion of cytokinesis. Phosphorylates the C-terminal repeat domain (CTD) of RNA polymerase II. [DNA-directed RNA polymerase] + ATP = ADP + H(+) + phospho-[DNA-directed RNA polymerase] One of the nine subunits forming the core-TFIIH basal transcription factor. Interacts with mcs2 and tfb3. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.