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Published online 12 January 2004

Nucleic Acids Research, 2004, Vol. 32, No. 1 298-306
© 2004 Oxford University Press

The guanine-rich fragile X chromosome repeats are reluctant to form tetraplexes

Petr Fojtík, Iva Kejnovská and Michaela Vorlícková*

Academy of Sciences of the Czech Republic, Institute of Biophysics, Královopolská 135, CZ-612 65 Brno, Czech Republic

*To whom correspondence should be addressed. Tel: +420 541517188; Fax: +420 541211293; Email: mifi{at}ibp.cz

Using circular dichroism spectroscopy, UV absorption spectroscopy and polyacrylamide gel electrophoresis, we studied conformational properties of guanine-rich DNA strands of the fragile X chromosome repeats d(GGC)n, d(GCG)n and d(CGG)n, with n = 2, 4, 8 and 16. These strands are generally considered in the literature to form guanine tetraplexes responsible for the repeat expansion. However, we show in this paper that the repeats are reluctant to form tetraplexes. At physiological concentrations of either Na+ or K+ ions, the hexamers and dodecamers associate to form homoduplexes and the longer repeats generate homoduplexes and hairpins. The tetraplexes are rarely observed being relatively most stable with d(GGC)n and least stable with d(GCG)n. The tetraplexes are exclusively formed in the presence of K+ ions, at salt concentrations higher than physiological, more easily at higher than physiological temperatures, and they arise with extremely long kinetics (even days). Moreover, the capability to form tetraplexes sharply diminishes with the oligonucleotide length. These facts make the concept of the tetraplex appearance in this motif in vivo very improbable. Rather, a hairpin of the fragile X repeats, whose stability increases with the repeat length, is the probable structure responsible for the repeat expansion in genomes.


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