Repair of aflatoxin B1 DNA adducts by the UvrABC endonuclease of Escherichia coli.

The repair by UvrABC endonuclease of two major adducts formed by aflatoxin B1 in DNA was found to be similar. Aflatoxin epoxide was used to generate the aflatoxin B1.N7-guanine adduct which can convert to aflatoxin B1-formamidopyrimidine adduct. The reaction of the aflatoxin B1 epoxide with DNA follows pseudo-first order kinetics. The DNA sequence-specific relative reactivity of the epoxide is the same as previously observed for aflatoxin B1 activated by liver microsomes, therefore strongly reinforcing the notion that aflatoxin B1 reacts with DNA through the epoxide intermediate. For the majority of lesion sites, a high affinity protein-DNA complex was formed from the UvrA and the UvrB proteins with similar efficiency to both adducts, and to pyrimidine dimers, and then nicks the DNA when UvrC was added. The two incisions are at the eighth phosphodiester moiety 5' and the sixth phosphodiester moiety 3' of a modified guanine nucleotide. Both incisions appeared to be concerted. For some sites, the DNA sequence can alter the relative incision efficiency up to 15-fold. However, the majority of these AFB1 lesion structures in most DNA sequences are similar with respect to recognition by this nucleotide excision repair enzyme. Therefore the observation that the aflatoxin B1.N7-guanine lesion is removed rapidly, while the aflatoxin B1-formamidopyrimidine lesion persists in the mammalian cell may have other mechanistic explanations.