a, Electrophoretic mobility shift assay monitors replacement of DSIF by CSB on the Pol II elongation complex. The gel was scanned in three different channels to reveal the elongation complex (via DNA), DSIF and CSB through different fluorescent labels. The experiment was repeated three times. Asterisks indicate the attachment of the fluorescent label. For gel source data, see Supplementary Fig. 1. b, CSB can replace DSIF during elongation and stimulate Pol II progression over an arrest site. The experiment was repeated three times. For gel source data, see Supplementary Fig. 1. c, Differences between EC* (left; PDB code: 6TED21) and the new ECTCR (right).