Molecular mechanism of DNA recognition by the alpha subunit of the Oxytricha telomere binding protein.

Interactions between telomeric DNA and the alpha subunit of the heterodimeric telomere binding protein of Oxytricha nova have been probed by Raman spectroscopy, CD spectroscopy, and nondenaturing gel electrophoresis. Telomeric sequences investigated include the Oxytricha 3' overhang, d(T4G4)2, and the related sequence dT6(T4G4)2, which incorporates a 5'-thymidylate leader. Corresponding nontelomeric isomers, d(TG)8 and dT6(TG)8, have also been investigated. Both d(T4G4)2 and dT6(T4G4)2 form stable hairpins that contain Hoogsteen G.G base pairs [Laporte, L., and Thomas, G. J., Jr. (1998) J. Mol. Biol. 281, 261-270]. The alpha subunit binds specifically and stoichiometrically to the dT6(T4G4)2 hairpin and alters its secondary structure by inducing conformational changes in the 5'-thymidylate leader without extensive disruption of G.G base pairing. Conversely, binding of the alpha subunit to d(T4G4)2 eliminates G.G pairing and unfolds the hairpin. DNA unfolding is accompanied by conformational changes affecting both the backbone and dG residues, as evidenced by Raman and CD spectra. Interestingly, the alpha subunit also forms complexes with the nontelomeric isomers, d(TG)8 and dT6(TG)8, evidenced by altered electrophoretic mobility in nondenaturing gels; however, Raman and CD spectra of complexes of the alpha subunit with nontelomeric DNA suggest no significant changes in backbone or deoxynucleoside conformations. Similarly, the alpha subunit binds to but does not appreciably alter the secondary structure of duplex DNA. The present results show that while the alpha subunit has the capacity to bind to Watson-Crick and different non-Watson-Crick motifs, DNA refolding is specific to the Oxytricha telomeric hairpin and the retention of G.G pairing is specific to the telomeric sequence incorporating the 5' leading sequence. A model is proposed for alpha subunit binding to telomeric DNA, and the physiological role of the alpha subunit in telomere organization is discussed.