Cdk9 activity on CTD and pSer5-CTD. (A) CTD and pSer5-CTD were loaded on SDS–PAGE. Phosphorylation by Cdk7/CycH shifts the CTD band upwards. (B) Kinase assay. Cdk9/CycT1 phosphorylates equally well CTD and pSer5-CTD (generated by previous incubation with Cdk7/CycH). No further incorporation of 32P could be detected incubating pSer5-CTD with Cdk7/CycH indicating that the previous reaction went to completeness.
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