The Dewar photoproduct of thymidylyl(3 '-> 5 ')-thymidine (Dewar product) exhibits mutagenic behavior in accordance with the "A rule"

In contrast to the highly mutagenic pyrimidine(6-4)pyrimidone photoproduct, its Dewar valence isomer (Dewar product) has low mutagenic potential and p roduces a broad range of mutations [LeClerc, J. E., Borden. A. & Lawrence, C. W. (1991) Proc. Natl. Acad. Sci. USA 88. 9685-9689]. To determine the or igin of the mutagenic property of the Dewar product, we used experimental N MR restraints and molecular dynamics to determine the solution structure of a Dewar-lesion DNA decamer duplex. This DNA decamer duplex (DW/GA duplex) contains a mismatched base pair between the 3' T residue of the Dewar lesio n (T6) and an opposed G residue (G15). The 3' T (T6) of the Dewar lesion fo rmed stable hydrogen bonds with the opposing G15 residue. However, the heli cal bending and unwinding angles of the DW/GA duplex were much larger than those of a second duplex that contains the Dewar lesion and opposing A15 an d A16 residues (DW/AA duplex). The DW/GA duplex showed poorer stacking inte ractions at the two bases of the Dewar product and at the adjacent A7.T14 b ase pair than did the DW/AA duplex. These structural features imply that no thermal stability or conformational benefit is obtained by incorporating a G instead of an A opposite the 3' T of the Dewar lesion. These properties may thus facilitate the preferential incorporation of an A in accordance wi th the A rule during translesion replication and lead to the low frequency of 3' T-->C mutations observed at this site.