2'-DEOXYISOGUANOSINE ADOPTS MORE THAN ONE TAUTOMER TO FORM BASE-PAIRSWITH THYMIDINE OBSERVED BY HIGH-RESOLUTION CRYSTAL-STRUCTURE ANALYSIS

The questions of whether different tautomeric forms of nucleic acid ba ses exist to any significant extent in DNA, or what their possible rol es in mutation may be, are under intense scrutiny. 2'-Deoxyisoguanosin e (iG) has been suggested to have a propensity to adopt the enol form. Isoguanine (also called 2-hydroxyadenine) can be found in oxidatively damaged DNA generated from treating DNA with a Fenton-type reactive o xygen-generating system and is known to cause mutation. We have analyz ed the three-dimensional structure of the DNA dodecamer d(CGC[iG]AATTT GCG) (denoted iG-DODE) by X-ray crystallography and NMR. The crystal s tructure of the iG-DODE complexed with the minor groove binder Hoechst 33342, refined to 1.4 Angstrom resolution, showed that the two indepe ndent iG.T base pairs in the dodecamer duplex adopt different (one in Watson-Crick and the other in wobble) conformations. The high-resoluti on nature of the structure also affords unprecedented clear informatio n about the conformation and interactions of the Hoechst drug. The Hoe chst 33342 binds in the narrow minor groove at the iGAATT site, with t he N-methylpiperazine ring near the iG4.T21 base pair. Three hydrogen bonds are found between the NH of the Hoechst ligand and T-O2 DNA atom s. In solution, the two iG.T base pairs in iG-DODE predominantly are i n the wobble form at 2 degrees C. At higher temperatures, another dupl ex form (likely involving the enol form of iG) is in slow exchange wit h the keto form and becomes significantly populated, reaching similar to 40% at 40 degrees C. Our data support the conclusion that iG pairs with T in a Watson-Crick configuration to a significant extent at phys iological temperature (37 degrees C), which may explain the facile inc orporation rate of T across from an iG during in vitro DNA replication .