Extended Data Fig. 1. Processing of cryo-EM data for SNAPc-containing Pol II PIC bound to U1 promoter. Related to Fig. 2.

a) Representative cryo-EM micrograph (out of 16, 854 in total) of the SNAPc-containing Pol II PIC bound to U1 promoter cryo-EM data collection. Scale bar – 240 Å. b) Representative 2D class averages of initially sorted datasets after merging. Adjacent to a well-defined PIC, clear signal for SNAPc is detected. c) Complete processing scheme. After initial clean-up procedures, particles representing SNAPc containing PIC were recovered as two sets. These particle sets were processed separately with respect to the promoter DNA state (CC/OC) and SNAPc occupancy. Final maps are coloured using the subunit color code in Fig. 1. The local resolution map indicate the resolution range of final maps (scale bar).