Figure 2

Analysis of protein composition of heparin-agarose purified fraction of Tat-associated CTD kinase. A , Heparin-agarose-purified fraction contains CDK7 but not CDK9. Fractions from the heparin-agarose column fractionation shown in Fig. 1 were analyzed by Western blotting with antibodies against CDK7, Cyclin H, PSTAIRE and CDK9. Fractions 22 to 24 which contain Tat-associated CTD hyperphosphorylating activity also contain CDK7 and cyclin H, but not CDK9 or PSTAIRE-like kinase. B, Tat-associated CTD kinase co-purifies with 35, 40 and 50 kDa protein bands. Fractions from the heparin-agarose column fractionation were resolved on 12% SDS PAGE and stained with silver. Protein bands of 35, 40, and 50 kDa that co-purify with the CTD kinase activity are marked by stars.