Tel1 activation by the MRX complex is sufficient for telomere length regulation but not for the DNA damage response inS. cerevisiae

Previous models suggested that regulation of telomere length in S. cerevisiae by Tel1(ATM) and Mec1(ATR) parallel the established pathways regulating the DNA damage response. Here we provide evidence that telomere length regulation differs from the DNA damage response in both the Tel1 and Mec1 pathways. We found that Rad53 mediates a Mec1 telomere length regulation pathway but is dispensable for Tel1 telomere length regulation, whereas in the DNA damage response Rad53 is regulated by both Mec1 and Tel1. Using epistasis analysis with a Tel1 hypermorphic allele, Tel1-hy909, we found that the MRX complex is not required downstream of Tel1 for telomere elongation but is required downstream of Tel1 for the DNA damage response. Since models that invoke a required end processing event for telomerase elongation are primarily based on the yeast pathways, our data call for a re-examination of the requirement for telomere end processing in both yeast and mammalian cells.