Fig. 3. A spectrum of telomeric states. A model showing three states at budding yeast telomeres. (A) A fully capped telomere that prevents checkpoint activation and repair pathways. It is capped by numerous telomere-binding proteins, indicated by T. (B) An uncapped telomere that has recruited the PIKK kinase Tel1p, the checkpoint protein Rad9p, and the MRX complex (encoded by MRE11, RAD50 and XRS2). This type of telomere is a weak inhibitor of cell division based on the fact that the TEL1-dependent response to unresected DSBs is weak (Usui et al., 2001) and that Tel1p overexpression causes transient arrest (Viscardi et al., 2003). Tel1p appears to be a potent activator of telomerases and contributes to telomere capping. (C) A resected, DSB-like telomere that has recruited the core members of the DNA damage checkpoint response, including MEC1, MEC3, RAD9, RAD17 and DDC1. This DSB-like telomere is a potent activator of cell cycle arrest but less efficient at recruiting telomerase.