Figure 3.

Models for steps in Tetrahymena telomere DNA synthesis and assembly of TER with TERT. (a) Schematic illustration of secondary structure of Tetrahymena TER bound to p65 (left) and to p65 and TERT (right). In the p65 bound TER, the pseudoknot is sequestered in a helix that prevents it from folding, but the TBE and loop 4 remain accessible for subsequent binding to TERT. (b) Schematic illustrating DNA exit pathway and interaction with Teb1, based on [13]. (c) Schematics illustrating template movement through the active site at beginning and end of synthesis of a telomere repeat. The template boundary element interacts with TERT to physically restrain the RNA so that residues beyond the template are not copied (right). A proposed sstDNA retention surface (SRS) within the CTE is colored as dark blue [56]. (d) Schematic illustrating the DNA hairpin proposed to form after translocation as part of the telomere repeat synthesis cycle [55]. This model assumes a large movement of the CTE (thumb) domain during translocation, indicated by the arrowheads connected by dotted line. The “hands” in the panels indicate the orientation of TERT in the panels relative to the views in figure 2.