TRF1-labeled chromatin reveals that telomeres form loops.

On page 161, Nikitina and Woodcock present the first isolation of telomeres complete with their chromatin protein complement.

Telomeres protect chromosome ends from degradation and shortening, but they must themselves be protected from repair systems that recognize free DNA ends. Images of telomere DNA, lacking its chromatin components, suggested that its protection might be enhanced by looping and insertion of the telomere's single-stranded overhang into the double-stranded telomeric repeats. Wrapping this loop up in chromatin proteins probably hides the overhang from agents that detect free ends, but because telomere chromatin is difficult to extract from the nuclear matrix, whether chromatin looping occurs was unknown.

The authors have conquered the chromatin extraction problem by using two types of blood cells that have fewer proteins gluing the chromatin to the matrix. After digesting away nontelomeric DNA, the authors labeled the remainder with biotinylated TRF1, a telomere-binding protein, thus identifying the telomeric chromatin. Like naked telomeric DNA, many of the TRF1-labeled structures formed loops.

As expected, the chromatin loop is smaller than that of the naked DNA due to nucleosomal packing. Nucleosomal repeat length is shorter on telomeres than on other DNA, but the authors did not note any changes in fiber morphology resulting from the difference. Future 3-D reconstructions will address this problem. An up-close look is also needed to determine how the overhang associates with the double-stranded DNA and how the nucleosome arrangement allows the loop to open and close during replication. ▪