T-loop assembly in vitro involves binding of TRF2 near the 3′ telomeric overhang

Mammalian telomeres contain a duplex TTAGGG-repeat tract terminating in a 3′ single-stranded overhang. TRF2 protein has been implicated in remodeling telomeres into duplex lariats, termed t-loops, in vitro and t-loops have been isolated from cells in vivo. To examine the features of the telomeric DNA essential for TRF2-promoted looping, model templates containing a 500 bp double-stranded TTAGGG tract and ending in different single-stranded overhangs were constructed. As assayed by electron microscopy, looped molecules containing most of the telomeric tract are observed with TRF2 at the loop junction. A TTAGGG-3′ overhang of at least six nucleotides is required for loop formation. Termini with 5′ overhangs, blunt ends or 3′ termini with non-telomeric sequences at the junction are deficient in loop formation. Addition of non-telomeric sequences to the distal portion of a 3′ overhang beginning with TTAGGG repeats only modestly diminishes looping. TRF2 preferentially localizes to the junction between the duplex repeats and the single-stranded overhang. Based on these findings we suggest a model for the mechanism by which TRF2 remodels telomeres into t-loops.