Crystallographic studies on damaged DNAs IV. N( 4)-methoxycytosine shows a second face for Watson-Crick base-pairing, leading to purine transition mutagenesis

To investigate the mutation mechanism of purine transitions in DNA damaged with methoxyamine, a DNA dodecamer with the sequence d(CGCAAATTmo⁴CGCG), where mo⁴C is 2′-deoxy-N⁴-methoxycytidine, has been synthesized and the crystal structure determined by X-ray analysis. The duplex structure is similar to that of the original undamaged B-form dodecamer, indicating that the methoxylation does not affect the overall DNA conformation. Electron density maps clearly show that the two mo⁴C residues form Watson–Crick-type base pairs with the adenine residues of the opposite strand and that the methoxy groups of mo⁴C adopt the anti conformation to N³ around the C⁴–N⁴ bond. For the pair formation through hydrogen bonds the mo⁴C residues are in the imino tautomeric state. Together with previous work, the present work establishes that the methoxylated cytosine residue can present two alternate faces for Watson–Crick base-pairing, thanks to the amino↔imino tautomerism allowed by methoxylation. Based on this property, two gene transition routes are proposed.