Tetrahelical forms of the fragile X syndrome expanded sequence d(CGG)(n) are destabilized by two heterogeneous nuclear ribonucleoprotein-related telomeric DNA-binding proteins.

Formations of hairpin and tetrahelical structures by the trinucleotide repeat sequence d(CGG)(n) might contribute to its expansion in fragile X syndrome. Here we show that tetraplex structures of d(CGG)(n) are destabilized by two mammalian heterogeneous nuclear ribonucleoprotein-related tetraplex telomeric DNA-binding and -stabilizing proteins, quadruplex telomeric DNA-binding protein 42 (qTBP42) (Sarig, G., Weisman-Shomer, P., Erlitzki, R., and Fry, M. (1997) J. Biol. Chem. 272, 4474-4482) and unimolecular quadruplex telomeric DNA-binding protein 25 (uqTBP25) (Erlitzki, R., and Fry, M. (1997) J. Biol. Chem. 272, 15881-15890). Blunt-ended and 3'-tailed or 3'- and 5'-tailed bimolecular tetraplex structures of d(CGG)(n) and guanine-sparse 20-/46-mer partial DNA duplex were progressively destabilized by increasing amounts of qTBP42 or uqTBP25 in time-dependent and ATP- or Mg(2+)-independent reactions. By contrast, tetraplex structures of telomeric and IgG sequences or guanine-rich double-stranded DNA resisted destabilization by qTBP42 or uqTBP25. Increased stability of tetraplex d(CGG)(n) in the presence of K(+) or Na(+) ions or at lowered reaction temperature diminished the destabilizing activity of uqTBP25. The contrasting stabilization of tetraplex telomeric DNA and destabilization of tetraplex d(CGG)(n) by qTBP42 and uqTBP25 suggested that sequence or structural differences between these tetraplexes might serve as cues for the differential stabilizing/destabilizing activities.