Identification of Escherichia coli mismatch-specific uracil DNA glycosylase as a robust xanthine DNA glycosylase.

The gene for the mismatch-specific uracil DNA glycosylase (MUG) was identified in the Escherichia coli genome as a sequence homolog of the human thymine DNA glycosylase with activity against mismatched uracil base pairs. Examination of cell extracts led us to detect a previously unknown xanthine DNA glycosylase (XDG) activity in E. coli. DNA glycosylase assays with purified enzymes indicated the novel XDG activity is attributable to MUG. Here, we report a biochemical characterization of xanthine DNA glycosylase activity in MUG. The wild type MUG possesses more robust activity against xanthine than uracil and is active against all xanthine-containing DNA (C/X, T/X, G/X, A/X and single-stranded X). Analysis of potentials of mean force indicates that the double-stranded xanthine base pairs have a relatively narrow energetic difference in base flipping, whereas the tendency for uracil base flipping follows the order of C/U > G/U > T/U > A/U. Site-directed mutagenesis performed on conserved motifs revealed that Asn-140 and Ser-23 are important determinants for XDG activity in E. coli MUG. Molecular modeling and molecular dynamics simulations reveal distinct hydrogen-bonding patterns in the active site of E. coli MUG that account for the specificity differences between E. coli MUG and human thymine DNA glycosylase as well as that between the wild type MUG and the Asn-140 and Ser-23 mutants. This study underscores the role of the favorable binding interactions in modulating the specificity of DNA glycosylases.