Activities and incision patterns of ABC excinuclease on modified DNA containing single-base mismatches and extrahelical bases.

ABC excision nuclease of Escherichia coli is a DNA repair enzyme that recognizes major helical distortions caused by bulky base adducts and incises on both sides of the adduct, thus removing the modified nucleotides in the form of a 12-13-base long oligomer. We tested the enzyme with substrates that contained unusual helical structures caused by single-base mismatches or one, three, or four extrahelical bases (loops). We find that the enzyme does not cut DNAs containing helical perturbations caused by these structures. However, when the mismatched or extrahelical bases are modified with 1-cyclohexyl-3-(2-morpholinoethyl) carbodiimide, a reagent specific for unpaired G and T residues, the enzyme incises at the modified nucleotides in the regular manner. In addition, we find that when mismatches and loops are located near pyrimidine dimers and (6-4) photoproducts they do not inhibit incision at the photoproducts by the excinuclease but sometimes affect the incision pattern. Our results indicate that ABC excinuclease may be a useful enzymatic reagent to probe the structural changes caused by mismatches and deletions in DNA and provide additional information on the requirements for incision by this repair enzyme.