DIFFERENCES AND SIMILARITIES IN THE REPAIR OF 2 BENZO[A]PYRENE DIOL EPOXIDE ISOMERS INDUCED DNA ADDUCTS BY UVRA, UVRB, AND UVRC GENE-PRODUCTS

We have determined the role of the uvrA, uvrB, and uvrC genes in Escherichia coli cells in repairing DNA damage induced by three benzo[a]pyrene diol epoxide isomers. Using the phi-X174 RF DNA-E. coli transfection system, we have found that BPDE-I or BPDE-II modified phi-X174 RF DNA has much lower transfectivity in uvrA, uvrB, and uvrC mutant cells compared to wild type cells. In contrast, BPDE-III modification of phi-X174 RF DNA causes much less difference in transfectivity between wild type and uvr- mutant cells. Moreover, BPDE-I- and -II-DNA adducts are much more genotoxic than are BPDE-III-DNA adducts. Using purified UVRA, UVRB, and UVRC proteins, we have found that these three gene products, working together, incise both BPDE-I- and BPDE-III-DNA adducts quantitatively and, more importantly, at the same rate. In general, UVRABC nuclease incises on both the 5' (six to seven nucleotides) and 3' (four nucleotides) sides of BPDE-DNA adducts with similar efficiency with few exceptions. Quantitation of the UVRABC incision bands indicates that both of these BPDE isomers have different sequence selectivities in DNA binding. These results suggest that although UVR proteins can efficiently repair both BPDE-I- and BPDE-III-DNA adducts, in vivo the uvr system is the major excision mechanism for repairing BPDE-I-DNA adducts but may play a lesser role in repairing BPDE-III-DNA adducts. It is possible the low lethality of BPDE-III-DNA adducts is due to less complete blockage of DNA replication. Alternatively, E. coli cells may have an alternative mechanism for repairing BPDE-III adducts that is not active in the repair of BPDE-I- or BPDE-II-DNA adducts.