Extreme mechanical diversity of human telomeric DNA revealed by fluorescence-force spectroscopy

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 116; no. 17; pp. 8350 - 8359
• Main Authors: Mitra, Jaba, Makurath, Monika A., Ngo, Thuy T. M., Troitskaia, Alice, Chemla, Yann R., Ha, Taekjip
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 23.04.2019
• Series: PNAS Plus
• Subjects: Biological Sciences; Change detection; Deoxyribonucleic acid; DNA; DNA - chemistry; DNA - ultrastructure; Energy transfer; Fluorescence; Fluorescence resonance energy transfer; Fluorescence Resonance Energy Transfer - methods; G-Quadruplexes; Guanine; Guanine - chemistry; Humans; Intermediates; Mechanical properties; Optical Tweezers; PNAS Plus; Repetitive Sequences, Nucleic Acid; Single-stranded DNA; Spectroscopy; Spectrum analysis; Telomere - chemistry; Telomere - ultrastructure; Thymine; Thymine - chemistry; Transcription; fluorescence resonance energy transfer; single-molecule biophysics; optical tweezers; G-quadruplex
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1815162116

G-quadruplexes (GQs) can adopt diverse structures and are functionally implicated in transcription, replication, translation, and maintenance of telomere. Their conformational diversity under physiological levels of mechanical stress, however, is poorly understood. We used single-molecule fluorescence-force spectroscopy that combines fluorescence resonance energy transfer with optical tweezers to measure human telomeric sequences under tension. Abrupt GQ unfolding with K⁺ in solution occurred at as many as four discrete levels of force. Added to an ultrastable state and a gradually unfolding state, there were six mechanically distinct structures. Extreme mechanical diversity was also observed with Na⁺, although GQs were mechanically weaker. Our ability to detect small conformational changes at low forces enabled the determination of refolding forces of about 2 pN. Refolding was rapid and stochastically redistributed molecules to mechanically distinct states. A single guanine-to-thymine substitution mutant required much higher ion concentrations to display GQ-like unfolding and refolded via intermediates, contrary to the wild type. Contradicting an earlier proposal, truncation to three hexanucleotide repeats resulted in a single-stranded DNA-like mechanical behavior under all conditions, indicating that at least four repeats are required to form mechanically stable structures.