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Mechanism of Copper Mediated Triple Helix Formation at Neutral pH in Drosophila Satellite Repeats

C. Paris ^* , F. Geinguenaud ^* , C. Gouyette , J. Liquier ^*  and J. Lacoste ^* 

^* Laboratoire de Biophysique Moléculaire, Cellulaire et Tissulaire (BioMoCeTi), Unité Mixte de Recherche, Centre National de la Recherche Scientifique 7033, Unité de Formation et de Recherche, Santé, Médecine et Biologie Humaine, Université Paris 13, 93017 Bobigny cedex, France; Université Pierre et Marie Curie, Case 138, 75252 Paris cedex 05, France; and Plate-forme Synthèse d'Oligonucléotides, Institut Pasteur, 75724 Paris cedex 15, France

Correspondence: Address reprint requests to J. Lacoste, Tel.: 33-1-48-38-73-91; Fax: 33-1-48-38-73-56; E-mail: jerl{at}ccr.jussieu.fr.

The highly repeated Drosophila melanogaster AAGAGAG satellite sequence is present at each chromosome centromere of the fly. We demonstrate here how, under nearly physiological pH conditions, these sequences can form a pyrimidine triple helix containing T·A-T and C^Cu·G-C base triplets, stabilized by Cu²⁺ metal ions in amounts mirroring in vivo concentrations. Ultraviolet experiments were used to monitor the triple helix formation at pH 7.2 in presence of Cu²⁺ ions. Triplex melting is observed at 23°C. Furthermore, a characteristic signature of triple helix formation was obtained by Fourier transform infrared spectroscopy. The stabilization of the C·G-C base triplets at pH 7.2 is shown to occur via interactions of Cu²⁺ ions on the third strand cytosine N3 atom and on the guanine N7 atom of the polypurine target strand forming C^Cu·G-C triplets. Under the same neutral pH conditions in absence of Cu²⁺ ions, the triple helix fails to form. Possible biological implications are discussed.

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