a, Analytical size-exclusion chromatography of EC* (left) and ECTCR (right) in the presence of RTF1. Peak fractions were analysed by SDS PAGE. While RTF1 elutes with EC* in stoichiometric amounts, it elutes with ECTCR sub-stoichiometrically, which is indicative of weaker association of RTF1 with ECTCR compared to EC*. The experiment with EC*-RTF1 was performed once and with ECTCR-RTF1 twice. For gel source data, see Supplementary Fig. 1. b, Modelling shows clashes between UVSSA and extended downstream DNA, and between SPT6 and extended upstream DNA, suggesting that some repositioning of DNA and/or SPT6 and UVSSA occurs when DNA is longer.