Repair of 4,5',8-trimethylpsoralen monoadducts and cross-links by the Escherichia coli UvrABC endonuclease.

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Abstract
Using an oligonucleotide model substrate, we observed two unusual mechanisms of UvrABC endonuclease in the repair of 4,5',8-trimethylpsoralen monoadducts and crosslinks. (i) UvrABC endonuclease usually incises a psoralen monoadduct only on the damaged strand. However, for one of the monoadducts we studied, incision on the complementary undamaged strand was also observed at a very low frequency, as though the adduct were on the thymine across from the damaged strand. Although the details of the erroneous incision are not yet known, such erroneous incision is potentially mutagenic. (ii) In cross-link repair, we observed that the UvrABC endonuclease incises the cross-linked DNA on either the furan side strand or the pyrone side strand. The incisions are not equally efficient. These data suggest that the structure of a psoralen cross-link, as seen by a repair enzyme, varies with the DNA sequence.