Endogenous Hot Spots of De Novo Telomere Addition in the Yeast Genome Contain Proximal Enhancers That Bind Cdc13.

DNA double-strand breaks (DSBs) pose a threat to genome stability and are repaired through multiple mechanisms. Rarely, telomerase, the enzyme that maintains telomeres, acts upon a DSB in a mutagenic process termed telomere healing. The probability of telomere addition is increased at specific genomic sequences termed sites of repair-associated telomere addition (SiRTAs). By monitoring repair of an induced DSB, we show that SiRTAs on chromosomes V and IX share a bipartite structure in which a core sequence (Core) is directly targeted by telomerase, while a proximal sequence (Stim) enhances the probability of de novo telomere formation. The Stim and Core sequences are sufficient to confer a high frequency of telomere addition to an ectopic site. Cdc13, a single-stranded DNA binding protein that recruits telomerase to endogenous telomeres, is known to stimulate de novo telomere addition when artificially recruited to an induced DSB. Here we show that the ability of the Stim sequence to enhance de novo telomere addition correlates with its ability to bind Cdc13, indicating that natural sites at which telomere addition occurs at high frequency require binding by Cdc13 to a sequence 20 to 100 bp internal from the site at which telomerase acts to initiate de novo telomere addition.

Regulation of telomere length by an N-terminal region of the yeast telomerase reverse transcriptase.

Telomerase is a reverse transcriptase that maintains chromosome integrity through synthesis of repetitive telomeric sequences on the ends of eukaryotic chromosomes. In the yeast Saccharomyces cerevisiae, telomere length homeostasis is achieved through negative regulation of telomerase access to the chromosome terminus by telomere-bound Rap1 protein and its binding partners, Rif1p and Rif2p, and positive regulation by factors such as Ku70/80, Tel1p, and Cdc13p. Here we report the identification of mutations within an N-terminal region (region I) of the yeast telomerase catalytic subunit (Est2p) that cause telomere lengthening without altering measurable catalytic properties of the enzyme in vitro. These telomerase mutations affect telomere length through a Ku-independent mechanism and do not alter chromosome end structure. While Tel1p is required for expression of the telomere-lengthening phenotype, Rif1p and Rif2p are not, suggesting that telomere overextension is independent of Rap1p. Taken together, these data suggest that specific amino acids within region I of the catalytic subunit of yeast telomerase play a previously unanticipated role in the response to Tel1p regulation at the telomere.