Extended Data Fig. 1: Protein preparation and nucleic acid scaffold design.

a, Quality of purified proteins used in this study. Purified proteins (0.9 µg) were run on 4–12% SDS–PAGE and stained with Coomassie blue. An asterisk demarcates SPT5 lacking an N-terminal region. b, Nucleic acid scaffold used for RNA extension assays, the ‘pause assay scaffold’. Template DNA is coloured in dark blue, non-template DNA is in light blue, and RNA is in red. c, Nucleic acid scaffold used for binding experiments and for cryo-EM analysis, the ‘HIV-1 pause scaffold’. Colours are the same as in b. d, SDS–PAGE analysis of fractions obtained from size-exclusion chromatography. The fractions used for cryo-EM analysis are marked. e, Quantification of the RNA extension assays shown in Fig. 1. The amount of elongated product was measured for each time point. Points are the mean of three independent experiments and error bars represent the standard deviation between experiments. f, Quantification of the RNA extension assays shown in Fig. 6. The amount of elongated product was measured for each time point. Points are the mean of three independent experiments and error bars represent the standard deviation between experiments.