Telomerase-dependent repeat divergence at the 3' ends of yeast telomeres

Yeast telomeres consist of approximately 300 nt of degenerate repeats with the consensus sequence G(2-3)(TG)(1-6). We developed a method for the amplification of a genetically marked telomere by PCR, allowing precise length and sequence determination of the G-rich strand including the 3' terminus. We examined wild-type cells, telomerase RNA deficient cells and a strain deleted for YKU70, which encodes for a protein involved in telomere maintenance and DNA double strand break repair. The 3' end of the G-rich strand was found to be at a variable position within the telomeric repeat. No preference for either thymine or guanine as the 3' base was detected. Comparison of telomere sequences from clonal populations revealed that telomeres consist of a centromere-proximal region of stable sequence and a distal region with differing degenerate repeats. In wild-type as well as yku70-Delta cells, variation in the degenerate telomeric repeats was detected starting 40-100 nt from the 3' end. Sequence divergence was abolished after deletion of the telomerase RNA gene. Thus, this region defines the domain where telomere shortening and telomerase-mediated extension occurs. Since this domain is much larger than the number of nucleo-tides lost per generation in the absence of telomerase, we propose that telomerase does not extend a given telomere in every cell cycle.