NEW TELOMERES IN YEAST ARE INITIATED WITH A HIGHLY SELECTED SUBSET OF TG(1-3) REPEATS

The creation of new telomeres was studied by generating a site-specific double-strand break in diploid strains of Saccharomyces cerevisiae that are unable to carry out homologous recombination. New telomere formation occurred approximately 1% of the time but only when (T2G4)13 was present proximal to the break site. About half of the healing events occurred at a number of 1- to 9-bp G or G,T sequences located as far as 128 bp distal to the T2G4 repeats. Surprisingly, in 16 events at sites ending in GTGG, the first TG1-3 nucleotides added always included either an 11- or a 13-bp sequence (GTGTGGGTGTG or GTGTGTGGGTGTG), after which each new telomere diverged into a less ordered TG1-3 pattern. Moreover, at 75% of the remaining addition sites, these same 11- or 13-bp sequences were found overlapping the junction between the chromosomal primer and the newly added sequences. We propose that short G,T sequences near an organizing sequence such as (T2G4)13 can act as primers to pair with the template RNA of a telomerase and add new sequences that are complementary to that RNA.