Role of DNA repair by (A)BC excinuclease and Ogt alkyltransferase in the final distribution of LacI-d mutations induced by N-butyl-N-nitrosourea in Escherichia coli

The alkyltransferase-like ybaZ gene product enhances nucleotide excision repair of O(6)-alkylguanine adducts in E. coli

O(6)-methylguanine adducts are potent pre-mutagenic lesions owing to their high capacity to direct mis-insertion of thymine when bypassed by replicative DNA polymerases. The strong mutagenic potential of these adducts is prevented by alkyltransferases such as Ada and Ogt in Escherichia coli that transfer the methyl group to one of their cysteine residues. Alkyl residues larger than methyl are generally weak substrates for reversion by alkyltransferases. In this paper we have investigated the genotoxic potential of the O(6)-alkylguanine adducts formed by ethylene and propylene oxide using single-adducted plasmid probes. Our work shows that the ybaZ gene product, a member of the alkyltransferase-like protein family, strongly enhances the repair by nucleotide excision repair of the larger O(6)-alkylguanine adducts that are otherwise poor substrates for alkyltransferases. The YbaZ protein is shown to interact with UvrA. This factor may thus enhance the efficiency of nucleotide excision repair in a way similar to the Transcription-Repair Coupling factor Mfd, by recruiting the UvrA(2).UvrB complex to the adduct site via its interaction with UvrA.

O6-pyridyloxobutylguanine adducts contribute to the mutagenic properties of pyridyloxobutylating agents

The tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) are potent carcinogens in animal models and likely human carcinogens. Both NNK and NNN can be activated to a pyridyloxobutylating agent. This alkylating agent contributes to the carcinogenic effects of NNK and NNN via the formation of miscoding DNA adducts. One of these adducts, O6-[4-oxo-4-(3-pyridyl)butyl]guanine (O6-pobG) has been characterized as a mutagenic adduct which is a substrate for the repair protein O6-alkylguanine-DNA alkyltransferase (AGT). Repair of O6-alkylguanine adducts by AGT protects cells from the mutagenic and carcinogenic effects of alkylating agents and is likely to play a similar role in shielding cells from the adverse effects of pyridyloxobutylating agents. Therefore, we examined the mutagenicity of the model pyridyloxobutylating agent, 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc), in Salmonella typhimurium YG7108 expressing hAGT. Expression of hAGT protected cells from NNKOAc-induced mutagenicity. Interestingly, hAGT did not shield cells from the toxicity of this agent. To confirm that the repair of O6-pobG was increased in the bacteria expressing hAGT, we measured levels of this adduct in NNKOAc-treated cultures. The levels of O6-pobG were lower in DNA from bacteria expressing hAGT. This work establishes an important role for O6-pobG in mediating the mutagenic, and possibly carcinogenic, effects of pyridyloxobutylating compounds.

DNA sequence context affects repair of the tobacco-specific adduct O(6)-[4-Oxo-4-(3-pyridyl)butyl]guanine by human O(6)-alkylguanine-DNA alkyltransferases

The repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) protects cells from the mutagenic and carcinogenic effects of alkylating agents by removing O(6)-alkylguanine adducts from DNA. Recently, we established that AGT protects against the mutagenic effects of pyridyloxobutylation resulting from the metabolic activation of the tobacco-specific nitrosamines (TSNA) 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and N-nitrosonornicotine by repairing O(6)-[4-oxo-4-(3-pyridyl)butyl]guanine (O(6)-pobG). There have been several epidemiologic studies examining the association between the I143V/K178R AGT genotype and lung cancer risk. Two studies have found positive associations, suggesting that AGT proteins differ in their repair of DNA damage caused by TSNA. However, it is not known how this genotype alters the biochemical activity of AGT. We proposed that AGT proteins may differ in their ability to remove large O(6)-alkylguanine adducts, such as O(6)-pobG, from DNA. Therefore, we examined the repair of O(6)-pobG by wild-type (WT) human, I143V/K178R, and L84F AGT proteins when contained in multiple sequence contexts, including the twelfth codon of H-ras, a mutational hotspot within this oncogene. The AGT-mediated repair of O(6)-pobG was more profoundly influenced by sequence context than that of O(6)-methylguanine. These differences are not the result of secondary structure (hairpin) formation in DNA. In addition, the I143V/K178R variant seems less sensitive to the effects of sequence context than the WT or L84F proteins. These studies indicate that the sequence dependence of O(6)-pobG repair by human AGT (hAGT) varies with subtle changes in protein structure. These data establish a novel functional difference between the I143V/K178R protein and other hAGTs in the repair of a toxicologically relevant substrate, O(6)-pobG.

Intra- and intercellular variations in the repair efficiency of O6-methylguanine, and their contribution to kinetic complexity

Following administration to rats of various doses of N-nitrosodimethylamine (NDMA), O(6)-methylguanine (O(6)-meG) was lost from the DNA of four tissues (liver, white blood cells, lymph nodes, bone marrow) over two, sharply demarcated phases with substantially differing repair rates. Repair during each phase followed approximately first-order kinetics in O(6)-meG, even after a high dose of NDMA which caused substantial depletion of O(6)-alkylguanine-DNA alkyltransferase (AGT), a suicide repair protein. This is compatible with rate-determining adduct repair being brought about by a distinct, minor pool of AGT molecules which is rapidly replenished by de novo AGT synthesis. Similar biphasic repair kinetics were also observed in HepG2 cells treated in vitro with NDMA. In this case, the first phase of repair was inhibited by alpha-amanitin, an inhibitor of RNA polymerase II-mediated transcription. However, no dependence on transcriptional activity was found when O(6)-meG repair in specific gene sequences with different transcriptional status in rat liver was examined, suggesting that the effects of alpha-amanitin in HepG2 cells did not reflect inhibition of preferential repair of transcribed sequences. Repair was also examined in rat liver hepatocytes and non-parenchymal cells separately after administration of NDMA at non-AGT depleting doses. Within each cell-population, the repair followed single phase, first-order kinetics, with adduct loss from AGT-rich hepatocytes being significantly faster than from the relatively AGT-deficient non-parenchymal cells. In conclusion, differences in the AGT content of different cell subpopulations in the liver (and probably in other tissues), as well as additional cellular factors affecting repair efficiency, appear to determine the observed variation in the kinetics of repair of O(6)-meG. The additional cellular factors involved appear not to be related to the transcriptional state of the sequences being repaired, but may reflect different states of chromatin condensation.