The origin recognition complex localizes to telomere repeats and prevents telomere-circle formation

Chromosome ends are maintained by telomere-repeat-binding factors (fRFs) that coordinate DNA end protection with telomere replication [1, 2]. The origin recognition complex (ORC) coordinates bidirectional DNA replication at most chromosomal sites, but it is also known to function in transcriptional silencing, heterochromatin formation, and sister-chromatid cohesion [3, 4]. We now show that ORC localizes to telomere repeats and contributes to telomere maintenance. We found that ORC subunits can be affinity purified with telomere-repeat DNA along with other components of the known "shelterin" complex. ORC subunits colocalized with telomere-repeat foci and colmmunoprecipitated with TRF2 but not TRF2 lacking its amino-terminal basic domain (TRF2 Delta B). ORC2 depletion or hypornorphic cell lines caused a loss of telomere-repeat signal intensity and the appearance of dysfunctional telomeres, including telomere-signalfree ends and telomere-repeat-containing double minutes. Two-dimensional agarose gel electrophoresis revealed that ORC2 depletion increased telomere circle formation, comparable to the overexpression of TRF2 Delta B. A similar increase in telomere circle formation was induced by hydroxyurea treatment, providing evidence that replication stress produces telomere circles. These findings suggest that ORC recruitment by TRF2 contributes to telomere integrity by facilitating efficient telomere DNA replication and preventing the generation of telomere-repeat-containing circles.