Recombination-mediated lengthening of terminal telomeric repeats requires the Sgs1 DNA helicase

The Saccharomyces cerevisiae SGS1 gene encodes a RecQ-like DNA helicase, hu man homologues of which are implicated in the genetic instability disorders , Bloom syndrome (BS), Rothmund-Thomson syndrome (RTS), and Werner syndrome (WS). Telomerase-negative yeast cells can recover from senescence via two recombinational telomere elongation pathways. The "type I" pathway generate s telomeres with large blocks of telomeric and subtelomeric sequences and s hort terminal repeat tracts. The "type II" pathway generates telomeres with extremely long heterogeneous terminal repeat tracts, reminiscent of the lo ng telomeres observed in telomerase-deficient human tumors and tumor-derive d cell lines. Here, we report that telomerase-negative (est2) yeast cells l acking SGS1 senesced more rapidly, experienced a higher rate of telomere er osion, and were delayed in the generation of survivors. The est2 sgs1 survi vors that were generated grew poorly, arrested in G(2)/M and possessed excl usively type I telomeres, implying that SGS1 is critical for the type II pa thway. The mouse WS gene suppressed the slow growth and G(2)/M arrest pheno type of est2 sgs1 survivors, arguing that the telomeric function of SGS1 is conserved. Reintroduction of SCS1 into est2 sgs1 survivors restored growth rate and extended terminal tracts by approximate to 300 bp. Both phenotype s were absolutely dependent on Sgs1 helicase activity. Introduction of an s gs1 carboxyl-terminal truncation allele with helicase activity restored gro wth rate without extending telomeres in most Eases, demonstrating that type II telomeres are not necessary for normal growth in the absence of telomer ase.