The removal of alkylation products from the DNA of Escherichia coli cells treated with the carcinogens N-ethyl-N-nitrosourea and N-methyl-N-nitrosourea: influence of growth conditions and DNA repair defects.

Cultures of Escherichia coli were treated with alkylnitrosoureas. The rates of removal of methylation and ethylation products from the DNA of strains defective in various repair pathways were compared with those of their respective wild-type strains. It was found that the removal of O6-methylguanine did not depend upon xth gene function or the uvr endonuclease. However, the rate of elimination of this product was markedly decreased in polA strains. O6-Ethylguanine (in contrast to its methyl analogue) was removed more slowly from the DNA of uvrA(-) than from that of uvrA(+) strains, indicating that the removal of O6-ethylguanine can be initiated by the uvr endonuclease. The composition of the medium in which methylated cells were resuspended following treatment with N-methyl-N-nitrosourea was also found to influence the rate at which O6-methylguanine was removed from the DNA of treated bacteria. No significant removal of this product from bacterial DNA occurred during treatment of cells in buffer, or when treated bacteria were resuspended in salts medium or in growth medium containing chloramphenicol. The results indicate that the elimination of O6-methylguanine, but not of 3-methyladenine, requires protein synthesis. Only very limited constitutive activity capable of removing O6-MeGua was detected.