Fluorescence resonance energy transfer as a probe for G-quartet formation by a telomeric repeat

The secondary structure of guanine-rich oligodeoxynucleotides has been inve stigated with fluorescent probes. Intramolecular folding of a telomeric oli gonucleotide into a quadruplex led to fluorescence resonance energy transfe r (FRET) between,a donor(fluorescein) and an acceptor (tetramethylrhodamine ) covalently attached to the 5' and 3' ends of the DNA, respectively. Depen ding on oligonucleoside length, quenching efficiency varied between 0.45 an d 0.72 at 20 degreesC. The conjugation of the dyes to the oligonucleotide h ad a limited but significant influence on the thermodynamics of G-quartet f ormation. Intramolecular folding was demonstrated from the concentration in dependence of fluorescence resonance energy transfer over a wide concentrat ion;range. Folding of the oligonucleotide was confirmed,by UV absorption UV melting, and circular dichroism;experiments. The folding of the G-quartet could be followed at concentrations as low as 100 pM. Fluorescence resonanc e energy transfer can thus be used to reveal the formation of multistranded DNA structures.