a, In vitro ubiquitylation of the complete Pol II–TCR complex by CRL4CSA. The experiment was repeated two times. For gel source data, see Supplementary Fig. 1. b, Tandem mass spectrometry fragment spectrum of the RPB1 peptide 1261-IMNSDENK(Gly-Gly)MQEEEE VVDKMDDDVFLR-1286. c, Structure of the ECTCR containing CRL4CSA. Two conformations related to targeting of the Pol II jaw domain (left) or CSB (right) for ubiquitylation are shown. RBX1 and the E2 enzyme–donor ubiquitin complex were modelled as described in the Methods.