Strand-specific postreplicative processing of mammalian telomeres

Telomeres are specialized nucleoprotein structures that stabilize the ends of linear eukaryotic chromosomes. In mammalian cells, abrogation of telomeric repeat binding factor TRF2 or DNA-dependent protein kinase (DNA-PK) activity causes end-to-end chromosomal fusion, thus establishing an essential role for these proteins in telomere function. Here we show that TRF2-mediated endcapping occurs after telomere replication. The postreplicative requirement for TRF2 and DNA-PKcs, the catalytic subunit of DNA-PK, is confined to only half of the telomeres, namely, those that were produced by leading-strand DNA synthesis. These results demonstrate a crucial difference in postrepticative processing of telomeres that is linked to their mode of replication.