Termination of DNA synthesis in vitro at apurinic sites but not at ethyl adducts on the template.

The effects of DNA lesions produced by the carcinogenic alkylating agents ethylnitrosourea and diethylsulfate on the extent of DNA synthesis have been studied in a system utilizing circular single-stranded phiX174 DNA as template and a 392-base restriction fragment as primer with E. coli polymerase I (Klenow fragment). Apurinic sites produced by loss of unstable ethylated bases from the template terminate DNA synthesis at the first such site encountered, but ethyl adducts at most, if not all, locations permit readthrough.