FIG. 5.

P-TEFb phosphorylation of SPT5. (A) Baculovirus-produced and purified proteins including CDK9 (lanes 1 and 6), CDK9 and cyclin T1 (lanes 2 and 7), CDK9 and cyclin T2a (lanes 3 and 8), and CDK9 and cyclin T2b (lanes 4 and 8) were assayed in in vitro kinase assays in the absence of the GST-SPT5 substrate (lanes 1 to 4), the GST-SPT5 substrate was assayed alone (lane 5), or the GST-SPT5 substrate was added along with the different CDK9 preparations (lanes 6 to 9). (B) The baculovirus-produced and purified preparations of CDK9 (lane 1), CDK9 and cyclin T1 (lane 2), CDK9 and cyclin T2a (lane 3), and CDK9 and cyclin T2b (lane 4) were assayed in Western blot analysis using antibodies directed against CDK9 (top panel), cyclin T1 (middle panel), or cyclin T2 (lower panel).