DNA‐end capping by the budding yeast transcription factor and subtelomeric binding protein Tbf1

Telomere repeats in budding yeast are maintained at a constant average length and protected (‘capped’), in part, by mechanisms involving the TG 1−3 repeat‐binding protein Rap1. However, metazoan telomere repeats (T 2 AG 3 ) can be maintained in yeast through a Rap1‐independent mechanism. Here, we examine the dynamics of capping and telomere formation at an induced DNA double‐strand break flanked by varying lengths of T 2 AG 3 repeats. We show that a 60‐bp T 2 AG 3 repeat array induces a transient G2/M checkpoint arrest, but is rapidly elongated by telomerase to generate a stable T 2 AG 3 /TG 1–3 hybrid telomere. In contrast, a 230‐bp T 2 AG 3 array induces neither G2/M arrest nor telomerase elongation. This capped state requires the T 2 AG 3 ‐binding protein Tbf1, but is independent of two Tbf1‐interacting factors, Vid22 and Ygr071c. Arrays of binding sites for three other subtelomeric or Myb/SANT domain‐containing proteins fail to display a similar end‐protection effect, indicating that Tbf1 capping is an evolved function. Unexpectedly, we observed strong telomerase association with non‐telomeric ends, whose elongation is blocked by a Mec1‐dependent mechanism, apparently acting at the level of Cdc13 binding.