Structural characterisation of the bromouracil.guanine base pair mismatch in a Z-DNA fragment

Abstract

The deoxyoligonucleotide d( _Br U-G-C-G-C-G) was crystallised at pH 8.2 and its structure analysed by X-ray diffraction. The unit cell, of dimensions a=17.94, b=30.85, c=49.94°A contains four DNA duplexes in space group P2 _l 2 _l 2 _l . The duplexes are in the Z conformation, with four Watson-crick G.C base pairs and two ^Br U.G base pairs. The structure was refined to an R factor of 0.16 at a resolution of 2.2°A with 64 solvent molecules located. The ^Br U.G base pair mismatch is of the wobble type, with both bases in the major tautomer fornuand hydrogen bonds linking 0–2 of ^Br U with N-l of G and N3 of ^Br U with 0–6 of G. There is no indication of the presence of ionised base pairs, in spite of the high pH of crystallisation. The results are discussed in terms of the mutagenic properties of 5- bromouracil.