3,N-4-ETHENOCYTOSINE, A HIGHLY MUTAGENIC ADDUCT, IS A PRIMARY SUBSTRATE FOR ESCHERICHIA-COLI DOUBLE-STRANDED URACIL-DNA GLYCOSYLASE AND HUMAN MISMATCH-SPECIFIC THYMINE-DNA GLYCOSYLASE

Exocyclic DNA adducts are generated in cellular DNA by various industr ial pollutants such as the carcinogen vinyl chloride and by endogenous products of lipid peroxidation. The etheno derivatives of purine and pyrimidine bases 3,N-4-ethenocytosine (epsilon C), 1,N-6-ethenoadenine (epsilon A), N-2,3-ethenoguanine, and 1,N-2-ethenoguanine cause mutat ions. The epsilon A residues are excised by the human and the Escheric hia coil 3-methyladenine-DNA glycosylases (ANPG and AlkA proteins, res pectively), but the enzymes repairing epsilon C residues have not yet been described. We have identified two homologous proteins present in human cells and E. coil that remove epsilon C residues by a DNA glycos ylase activity. The human enzyme is an activity of the mismatch-specif ic thymine-DNA glycosylase (hTDG). The bacterial enzyme is the double- stranded uracil-DNA glycosylase (dsUDG) that is the homologue of the h TDG. In addition to uracil and epsilon C-DNA glycosylase activity, the dsUDG protein repairs thymine in a G/T mismatch. The fact that epsilo n C is recognized and efficiently excised by the E. coil dsUDG and hTD G proteins in vitro suggests that these enzymes may be responsible for the repair of this mutagenic lesion in vivo and be important contribu tors to genetic stability.