C are repaired by the AfAlkA base excision repair glycosylase of Archaeoglobus fulgidus, suggesting a different repair mechanism for these lesions in the third domain of life. In addition, AfAlkA was found to effect a robust excision of 1,N 6 -ethenoadenine. We present a high-resolution crystal structure of AfAlkA, which, together with the characterization of several sitedirected mutants, forms a molecular rationalization for the newly discovered base excision activity.

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