A novel uracil-DNA glycosylase with broad substrate specificity and an unusual active site

Uracil-DNA glycosylases (UDGs) catalyse the removal of uracil by flipping it out of the double helix into their binding pockets, where the glycosidic bond is hydrolysed by a water molecule activated by a polar amino acid. Interestingly, the four known UDG families differ in their active site make-up. The activating residues in UNG and SMUG enzymes are aspartates, thermostable UDGs resemble UNG-type enzymes, but carry glutamate rather than aspartate residues in their active sites, and the less active MUG/TDG enzymes contain an active site asparagine. We now describe the first member of a fifth UDG family, Pa-UDGb from the hyperthermophilic crenarchaeon Pyrobaculum aerophilum, the active site of which lacks the polar residue that was hitherto thought to be essential for catalysis. Moreover, Pa-UDGb is the first member of the UDG family that efficiently catalyses the removal of an aberrant purine, hypoxanthine, from DNA. We postulate that this enzyme has evolved to counteract the mutagenic threat of cytosine and adenine deamination, which becomes particularly acute in organisms living at elevated temperatures.