Site-specific effect of thymine dimer formation on dAn.dTn tract bending and its biological implications

dAn.dTn sequences, otherwise known as A tracts, are hotspots for cis-syn thymine dimer formation and deletion mutations induced by UV light. Such A tracts are also known to bend DNA, suggesting that some biological effects of UV light might be related to the distinctive structure and properties of cis-syn dimer-containing A tracts. To investigate the effect of thymine dimer formation on A-tract bending multimers of all possible dimer monoadducts of a dA6.dT6-containing decamer known to bend DNA were prepared along with multimers of a dimer-containing 21-mer of heterogeneous sequence. The characteristic anomalous electrophoretic behavior of the phased A-tract multimers was essentially abolished by dimer formation at the center of the A tract and was only slightly reduced by dimer formation at the ends. These effects are attributed to disruption of the A-tract structure at the site of the dimer, resulting in intact A tracts of reduced length and, hence, reduced bending. This model was suggested by the ability to formulate the estimated bend angles of the dimer-containing A tracts as approximately equal to the sum of the bend angles induced by the dimer and the remaining intact portion of the A tract. Contrary to a previous experimental study that concluded that the thymine dimer bends DNA by approximately 30 degrees, the dimer was determined to bend DNA by only approximately 7 degrees. Reduction of the bending of a DNA sequence by dimer formation may have a number of unpredicted and important biological consequences.