Fig. 4. Reconstitution of complete telomere end replication.

a, Telomerase products are unlabelled (except in the T-body marker lanes), and C-strands are labelled with [α-³²P]dCTP. Time between initiation of the telomerase reaction and addition of 57 nM CST–Polα–primase (WT CST) is indicated at top. Control lanes are reactions identical to WT CST except containing no CST, no telomerase, no ribonucleotides (rNTPs) or no 3×TEL primer for telomerase. In the two right-hand sets of lanes, 57 nM g2.1 CST, a DNA-binding-defective mutant of CST, is substituted for WT CST. The uncropped gel is shown in Supplementary Fig. 1, and an experiment with intermediate time points is shown in Extended Data Fig. 10d. b, Model for reconstituted telomere replication. (1) Telomerase (RNA template in orange) binds to the 3×TEL DNA primer (grey) and extends it with telomeric repeats (grey rectangles). (2) Telomerase dissociates, and CST–Polα–primase binds to telomeric repeats and begins RNA primer synthesis with ATP. (3) Primase synthesizes RNA primer (~8 nt). (4) Template–primer pair is handed off to Polα, which catalyses C-strand DNA synthesis (blue bar). The continued presence of CST during extension is unknown.