Western blot of an in vitro kinase assay using CDK12/cyclin K and RNAPII complemented with PAF1C WT, CDC73 absent (ΔCDC73), or the CDC73 mutant variants shown in (D). ΔATP, no ATP added. NTD, N-terminal domain; S2+S5, Ser2P + Ser5P. Red arrows indicate phosphorylation products.
