Fig. 1. Six classes of functional telomere in budding yeast. Natural telomeres of budding yeast are illustrated in A and B (Pryde and Louis, 1997) and other types of functional telomere are illustrated in C to F. The data and colouring scheme are from the website of Ed Louis (http://www.le.ac.uk/ge/ejl12/research/telostruc/EndsSmall.html). (A) Y' telomeres contain the three major repetitive sequences found at budding yeast telomeres: G-rich, Y' and X repeats. G-rich and X repeats are found at all telomeres. The G-rich repeats are the product of telomerase activity and are approximately 300 bp in wild-type budding yeast strains. X repeats are based on a 473 bp core sequence that contains an ARS (autonomously replicating sequence) consensus sequence, the binding site of the origin recognition complex and a separate Abf1 (ARS binding factor 1) binding site (Pryde and Louis, 1997; Pryde and Louis, 1999). Y' repeats are considerably larger than X repeats, with two predominant sizes of 5.2 and 6.7 kb (Lundblad and Blackburn, 1993). Y' repeats also contain ARS consensus sequences and Abf1 binding sites and are therefore potential origins of replication (Pryde and Louis, 1997). In addition, Y' repeats encode a functional helicase (Yamada et al., 1998). (B) X telomeres contain only G-rich and X repeats (C) In the absence of telomerase, or if telomere capping is defective, cells enter crisis and generate survivors. Type I survivors lose most of the G-rich repeats but amplify Y' repeats by recombination-dependent mechanisms. (D,E) In the absence of telomerase, Type II survivors contain highly lengthened G-rich repeats that have been maintained by recombination-dependent mechanisms. (F) If a DSB is induced close to a G-rich telomere seed sequence a telomere can be formed de novo.