The effects of three widely differing chemical carcinogens, 4-nitroquinoline 1-oxide, Adriamycin, and ethyleneimine, on DNA replication were studied by pulse labeling of DNA with [3H]thymidine and sedimentation analysis with alkaline sucrose gradients. At doses that reduced the rate of DNA synthesis to 30 to 60% of control values, only ethyleneimine produced damage that resulted in lower molecular weights of parental DNA. All three chemicals inhibited replicon initiation, but to differing

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... s. Inhibition of replicon initiation was the first clearly identified effect of 4-nitroquinoline 1-oxide and was the main cause of inhibition of DNA synthesis. Ethyleneimine caused severe inhibition of replicon initiation, but blocks to chain elongation also contributed significantly to the inhibition of overall DNA synthesis. Adriamycin affected replicon initiation to a small but significant extent; the primary cause of inhibition of DNA synthesis by this drug was a slowing of the rate of chain elongation. These results indicate that inhibition of replicon initiation is an important mechanism for the action of DNA-damaging agents in mammalian cells and strengthen the concept that control of DNA replication depends on the structural integrity of a chromosomal subunit that consists of several replicons.