The beta subunit of Oxytricha telomere-binding protein promotes G-quartet formation by telomeric DNA

Telomeres, the ends of linear chromosomes, typically consist of tandem repeats of simple G-rich sequences. At high concentrations, single-stranded telomeric DNA can form dimers and tetramers involving G-quartets. We show that under physiological conditions, the beta subunit of the Oxytricha telomere-binding protein greatly accelerates G-quartet formation. The reaction occurs with oligonucleotides ending in the Oxytricha (T4G4T4G4) and Tetrahymena (T2G4T2G4) telomeric sequences; the sequence preceding these telomeric repeats can be nontelomeric, single-, or double-stranded. Protein deletion analysis indicates that the carboxy-terminal highly basic domain of the beta subunit, which is dispensible for telomeric complex formation, is sufficient for mediating G-quartet formation. The finding that a telomeric protein acts as a molecular chaperone for G-quartet formation provides a strong argument that such DNA structures exist in vivo at chromosome telomeres.