The SP lesion induces structural changes within the DNA. (a) The Fo − Fc electron-density map (green mesh) is shown prior to inclusion of the SP and complementary adenines in the structural model contoured at 2.5σ, with the final refined model for the lesion shown as a stick model. The electron density for the covalent bonding between the thymines is clearly evident. (b) Stick renderings of nine base pairs are shown in stereo for the SP-containing duplex DNA in green, the SP lesion in blue and the non-SP-containing DNA duplex in magenta. The first three base pairs within each structure superimpose well. Deviations in the two structures are then apparent beyond these first base pairs. The rough positioning of a spline through the phosphodiester backbone is indicated by red lines for the SP-containing duplex and by an orange line for the non-SP-containing duplex. In this view, the significant widening of the minor groove of the SP-containing DNA is apparent. (c). Stick renderings are shown in stereo for the base pairs (green) between the two SP lesions (blue) within our 16 bp oligonucleotide superimposed on the equivalent region of the structure of a single SP lesion (orange) found within an oligonucleotide complexed with B. stearothermophilus DNA polymerase I. The second SP lesion in our structure (not shown) would be on the strand complementary to that including A8 and T9.