SGS1 is required for telomere elongation in the absence of telomerase

In S. cerevisiae, mutations in genes that encode telomerase components, suc h as the genes EST1, ESM, EST3, and TLC1, result in the loss of telomerase activity in vivo [1-3], Two telemerase-independent mechanisms can overcome the resulting senescence. Type I survival is characterized by amplification of the subtelomeric Y ' elements with a short telomere repeat tract at the terminus [4], Type II survivors arise through the abrupt addition of long tracts of telomere repeats [4-6], Both mechanisms are dependent on RAD52 [4 , 5] and on either RAD50 or RAD51 [6, 7], We show here that the telomere el ongation pathway in yeast (type II) is dependent on SGS1, the yeast homolog of the gene products of Werner's (WRN) [8] and Bloom's (BLM) [9] syndromes . Survival in the absence of SGS1 and EST2 is dependent upon RAD52 and RAD5 1 but not RAD50, We propose that the RecQ family helicases are required for processing a DNA structure specific to eroding telomeres.