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Enzymatic capture of an extrahelical thymine in the search for uracil in DNA
Jared B. Parker1, Mario A. Bianchet2, Daniel J. Krosky1, Joshua I. Friedman1, L. Mario Amzel2 & James T. Stivers1
Correspondence to: James T. Stivers1 Correspondence and requests for materials should be addressed to J.T.S. (Email: jstivers@jhmi.edu).
The enzyme uracil DNA glycosylase (UNG) excises unwanted uracil bases in the genome using an extrahelical base recognition mechanism. Efficient removal of uracil is essential for prevention of C-to-T transition mutations arising from cytosine deamination, cytotoxic UA pairs arising from incorporation of dUTP in DNA, and for increasing immunoglobulin gene diversity during the acquired immune response. A central event in all of these UNG-mediated processes is the singling out of rare U
A or U
G base pairs in a background of approximately 109 T
A or C
G base pairs in the human genome. Here we establish for the human and Escherichia coli enzymes that discrimination of thymine and uracil is initiated by thermally induced opening of T
A and U
A base pairs and not by active participation of the enzyme. Thus, base-pair dynamics has a critical role in the genome-wide search for uracil, and may be involved in initial damage recognition by other DNA repair glycosylases