Figure 5.

Cryo–electron microscopy (EM) structure of Tetrahymena telomerase and model of human TERT–t/PK. (a) Front view of the 9.4-Å cryo-EM map with the RNP core (blue), TEB (gold), CST (tan), and p50 (red). Arrows indicate location of protein C-terminus (or N-terminus for p50) and TR SL2 determined by Fab labeling and MS2 coat protein labeling, respectively, in negative stain EM (70). (b) Same color designation and labeling scheme as panel a but with pseudoatomic models of the RNP core, TEB trimer of OB folds, and CST trimer of OB folds. Additional domains of Teb1, Teb2, and p45, not visible in the cryo-EM map, are shown as crystal structures [Teb1A (PDB ID 3U4V), Teb1B (PDB ID 3U4Z), p45C (PDB ID 5DFN)], homology models (Teb2C based on PDB ID 1DPU), or ovals (70). (c) Schematic of Tetrahymena telomerase, 180° rotation from panel b, illustrating DNA bound to template and exit path. (d) Schematic illustrating movement of the template through the active site for one round of telomere repeat synthesis, and template boundary definition by the TBE. (e) Pseudoatomic model of Tetrahymena telomerase catalytic core, showing the path of TR on TERT. Inset shows schematic of the t/PK. TR elements in panels e and f are colored as in Figure 2. (f) Model of model of human telomerase catalytic core, based on the model of the human t/PK (171, 188), medaka CR4/5 with TRBD (PDB ID 4O26) (63), and the positions of homologous domains of Tetrahymena TR in the cryo-EM map. Inset shows schematic of the t/PK with TR elements colored as in the model. Abbreviations: CR4/5, conserved regions 4 and 5; CTE, C-terminal extension; OB, oligonucleotide/oligosaccharide binding; RNP, ribonucleoprotein; RT, reverse transcriptase; sstDNA, single-stranded telomere repeat DNA; STE, stem terminus element; TBE, template boundary element; TEN, TERT-essential N-terminal domain; TERT, telomerase reverse transcriptase; t/PK, template/pseudoknot; TR, telomerase RNA.