CROSS-LINKING OF SRF TO THE C-FOS SRE CARG BOX GUANINES USING PHOTO-ACTIVE THIOGUANINE OLIGODEOXYNUCLEOTIDES

6-Thioguanine (thioG) was chemically incorporated into 25-base oligode oxynucleotides encoding the c-fos serum response element (SRE) at posi tions corresponding to each guanine of the CArG box, which only slight ly impaired DNA binding by the Serum Response Factor (SRF). Upon expos ure to long wavelength UV light each thioG-containing SRE could be cro sslinked to SRF, with efficiencies ranging from <1 to 25% of the compl ex depending on the position of thioG in the SRE and on the UV source used. Crosslinking was strongest to the 3' side of the CArG box, and t o the outer rather than the inner CArG box guanines, consistent with h ydrogen bonds formed between SRF and the outer guanines in the crystal structure [Pellegrini et al., Nature 376, 490, 1995]. The crosslinked product was found to be chemically unstable. Possible mechanisms of c rosslink formation are discussed. (C) 1996 Academic Press, Inc.