SEMINAR 2025

Abstract

Telomeres, the DNA–protein caps at chromosome ends, are maintained by two molecular systems working on opposite strands. Using cryogenic electron microscopy and biochemical reconstitution, we show how the CST complex organizes DNA polymerase-α–primase into a preinitiation complex optimized for RNA–DNA primer synthesis, positioning the active sites in sequence and enforcing a defined template-length requirement for telomere C-strand fill-in. On the complementary telomere G-strand, we identify Replication Protein A (RPA) as a second telomerase processivity factor, distinct from the TPP1–POT1 recruitment complex. AlphaFold modeling, separation-of-function mutants, and cell assays reveal that RPA directly increases the number of DNA repeats added per telomerase engagement, and that disease-linked telomerase variants impair this boost. These findings define the physical basis for a division of labor at chromosome ends.