A protein from Tetrahymena thermophila that specifically binds parallel-stranded G4-DNA.

G4-DNA is a parallel, four-stranded structure mediated by tetrads of hydrogen-bonded guanines (G-quartets). An abundant protein called Tetrahymena G4 binding protein (TGP) that binds to an intermolecular, quadruplex form of d(TTGGGGTTGGGGTTGGGGTTGGGG) under physiological salt conditions has been identified in cellular extracts from the ciliated protozoan Tetrahymena thermophila. In binding competition experiments, molecules capable of forming G4 structures compete for binding to TGP, but non-G4-forming molecules and r(U2G4)4 do not. TGP binding also requires a single-stranded region adjacent to the G4 structure. During the course of this study, it was determined that Mg2+ facilitates the formation of parallel-stranded G4-DNA structures and that high oligonucleotide concentrations are not required to drive formation of these structures. In addition, G4-DNA and TGP/G4-DNA complexes form readily under physiological salt conditions. These data support the proposal that G4-DNA structures exist in vivo.