Telomere recombination preferentially occurs at short telomeres in telomerase-null type II survivors

In telomerase negative yeast cells, Rad52-dependent recombination is activated to maintain telomeres. This recombination-mediated telomere elongation usually involves two independent pathways, type I and type II, and leads to generation of type I and type II survivors. It remains elusive whether the recombination-mediated telomere elongation prefers to take place on shorter or longer telomeres. In this study, we exploited the de novo telomere addition system to examine the telomere recombination event in telomerase negative cells. We show that recombination preferentially occurs on shorter rather than longer telomeres in both pre-survivors and established type II survivors. In type II survivors, the short VII-L telomeres could invade either terminal TG1-3 sequence or short tracts of TG1-3 sequence in subtelomeric Y'-X and Y'-Y' junction to initiate recombination. Unexpectedly, short VII-L telomere recombination still takes place in type II survivors lacking either Rad50 or Rad59, which are required for type II survivor generation in senescing telomerase-null cells. Our results support the notion that Rad50 and Rad59 are not essential for the maintenance of type II survivors once established.