Extended Data Fig. 7: Identification of histone H2A and H2B as subunits of the human telomerase holoenzyme.

a, Three-dimensional classification of the catalytic core (dataset 1), showing the presence of the unaccounted-for density (in yellow). The best class (boxed) has the most homogenous density, and was selected for the final refinement. Similar observations were made with the second dataset. b, The 8 Å catalytic core map (in grey) with the previously unmodelled density (in yellow)³ (left) and with the model of the catalytic core obtained from this work fitted into it (right). The density assigned as the histone H2A–H2B dimer coincided with the unmodelled density from the previous work. c, The refined catalytic core map, with hTR, TERT and histone H2A–H2B segmented in different colours. d, Top, interactions between CR4/5 and the histone H2A–H2B dimer in human telomerase. Bottom, electrostatic surface potential of the histone dimer and the positively charged surface used for interacting with the CR4/5. e, Nucleosome structure. The histone H2A–H2B dimer is coloured and oriented the same way as in d. This shows that the histone H2A–H2B dimer uses the same surface to bind both nucleosomal DNA and CR4/5 (PDB 1KX5²⁷). f, Purified histone H2A–H2B and CR4/5 RNA used for electrophoretic mobility shift assay in Fig. 3f. No RNA ladders were loaded with the CR4/5 RNA. g, Immunoblots of crude lysate of HEK 293T cells transfected with twin Strep-TERT and hTR expression constructs (input) and oligonucleotide elution from 2′-O-methyl purification (Fig. 3c). These samples were immunoblotted for Strep and histones H2A, H2B, H3 and H4. The presence of histones H2A, H2B, H3 and H4 was also confirmed by mass spectrometry (Supplementary Data 4). h, Immunoblots of crude lysate of HEK 293T cells (input) and oligonucleotide elution from 2′-O-methyl purification. These samples were immunoblotted for dyskerin, and histones H2A, H2B, H3 and H4. i, Structure of the Tetrahymena TRBD–CTE–p65 and stem loop 4 (SL4) (PDB 6D6V¹⁶). j, Structure of human TRBD–CTE–H2A–H2B and CR4/5. k, Quantification of the electrophoretic mobility shift assays shown in Fig. 3f for K_d determination. ‘Total fraction bound’ reflects the quantification of free probe depletion against total probe with increasing histone concentration. ‘Specific fraction bound’ reflects the quantification of the increasing discrete shifted complex band against total probe with increasing histone concentration. Points represent values from three independent replicates (Supplementary Fig. 1). l, Superposition of the histone octamer structure, with flexible histone tails removed, onto the histone H2A–H2B dimer bound to the human telomerase catalytic core in two different views (PDB 1KX5²⁷).