Chloroacetaldehyde-induced mutagenesis in Escherichia coli: The role of AlkB protein in repair of 3,N4-ethenocytosine and 3,N4-α-hydroxyethanocytosine

Etheno (ɛ) adducts are formed in reaction of DNA bases with various environmental carcinogens and endogenously created products of lipid peroxidation. Chloroacetaldehyde (CAA), a metabolite of carcinogen vinyl chloride, is routinely used to generate ɛ-adducts. We studied the role of AlkB, along with AlkA and Mug proteins, all engaged in repair of ɛ-adducts, in CAA-induced mutagenesis. The test system used involved pIF102 and pIF104 plasmids bearing the lactose operon of CC102 or CC104 origin (Cupples and Miller (1989) [17]) which allowed to monitor Lac+ revertants, the latter arose by GC → AT or GC → TA substitutions, respectively, as a result of modification of guanine and cytosine. The plasmids were CAA-damaged in vitro and replicated in Escherichia coli of various genetic backgrounds. To modify the levels of AlkA and AlkB proteins, mutagenesis was studied in E. coli cells induced or not in adaptive response. Formation of ɛC proceeds via a relatively stable intermediate, 3,N4-α-hydroxyethanocytosine (HEC), which allowed to compare repair of both adducts. The results indicate that all three genes, alkA, alkB and mug, are engaged in alleviation of CAA-induced mutagenesis. The frequency of mutation was higher in AlkA-, AlkB- and Mug-deficient strains in comparison to alkA+, alkB+, and mug+ controls. Considering the levels of CAA-induced Lac+ revertants in strains harboring the pIF plasmids and induced or not in adaptive response, we conclude that AlkB protein is engaged in the repair of ɛC and HEC in vivo. Using the modified TTCTT 5-mers as substrates, we confirmed in vitro that AlkB protein repairs ɛC and HEC although far less efficiently than the reference adduct 3-methylcytosine. The pH optimum for repair of HEC and ɛC is significantly different from that for 3-methylcytosine. We propose that the protonated form of adduct interact in active site of AlkB protein.