Telomerase-independent lengthening of yeast telomeres occurs by an abrupt Rad50p-dependent, Rif-inhibited recombinational process

Type II survivors arise in Saccharomyces cells lacking telomerase by a reco mbinational pathway that results in very long and heterogeneous length telo meres. Here we show that type II telomeres appeared abruptly in a populatio n of cells with very short telomeres. Once established, these long telomere s progressively shortened. Short telomeres were substrates for rare, one-st ep lengthening events. The generation of type II survivors was absolutely R ad50p dependent. In a telomerase-proficient cell, the telomere-binding Rif proteins inhibited telomerase lengthening of telomeres. In a telomerase-def icient strain, Rif proteins, especially Rif2p, inhibited type II recombinat ion. These data argue that only short telomeres are substrates for type II recombination and suggest that the donor for this recombination is not a ch romosomal telomere.