Evidence for a New Step in Telomere Maintenance

The strand of telomeric DNA that runs 5′–3′ toward a chromosome end is typically G rich. Telomerase-generated G tails are expected at one end of individual DNA molecules. Saccharomyces telomeres acquire TG 1–3 tails late in S phase. Moreover, the telomeres of linear plasmids can interact when the TG 1–3 tails are present. Molecules that mimic the structures predicted for telomere replication intermediates were generated in vitro. These in vitro generated molecules formed telomere–telomere interactions similar to those on molecules isolated from yeast, but only if both ends that interacted had a TG 1–3 tail. Moreover, TG 1–3 tails were generated in vivo in cells lacking telomerase. These data suggest a new step in telomere maintenance, cell cycle–regulated degradation of the C 1–3A strand, which can generate a potential substrate for telomerase and telomere-binding proteins at every telomere.