UV light radiation damages DNA and leads to mutation and cancer. The (6-4) adduct is one of the classes of UV-induced DNA photoproducts. The (6-4) adduct is both highly mutagenic and highly specific with respect to the mutations induced (3`` T→C transition) in SOS-induced cells of E.coli. However the nearest neighboring residues of the (6-4) adducts have different effects on the replicating error frequency at this site. The (6-4) adducts flanked by guanines caused higher frequency of the replicating error than those flanked by adenines.
In order to understand the effects of the flanking residues on the mutational properties of the (6-4) adducts, we explored the relationship between the thermal stability of the (6-4)lesion containing duplex and sequence-dependent replicating error frequency by using a combination of both NMR spectroscopy and UV melting experiments. NMR spectra indicate that at all base pairs, (6-4) adduct flanked by G`` was more stabilized than that flanked by A`` when an G opposite the 3`` of (6-4) adduct was substituted for A. Specially, the (6-4) adduct flanked by G ?s of which the 5?side is the most stabilized. And at 37℃ the difference of the free energy changes upon formation (ΔΔG°) of the (6-4) adduct flanked by A`` was -1.26kcal/mol, and was -1.887 kcal/mol in the (6-4) adduct flanked by G``s. This also indicates that the extent of the stabilization of the (6-4) adduct flanked by G`` are larger than flanked by A``. Therefore, because of the themal stability of the (6-4) adduct depending on nearest neighboring residues we conclude that the (6-4) adduct flanked by A`` led to 57% 3→C transition which is significantly lower than that flanked by G`` of 85%.