Figure 3.

RNA polymerase II CTD is phosphorylated at residue Thr4. (A) Western blot analysis of HeLa cell extracts with Rpb1-specific mAb (POL 3/3) and the Thr4-P-specific mAb (6D7). II0 and IIA designate the hyperphosphorylated and hypophosphorylated forms of the large subunit Rpb1 of Pol II. (B) Extracts of mouse (MEF, NIH3T3) and human cell lines (Raji, U2OS, HEK293), or S. pombe and S. cerevisiae were analysed with mAbs specific for the various phospho-residues in CTD described in Figure 2A. (C, D) Immunoprecipitation (IP) experiments with phospho-specific CTD mAbs for Thr4-P, Ser2-P, Ser5-P, and Ser7-P in extracts of HeLa cells. Analysis of western blots with indicated mAbs. SN, supernatant; IgG1, IgG2a, and IgG2b are isotype controls.