Different rate-limiting steps in excision repair of ultraviolet- and N-acetoxy-2-acetylaminofluorene-damaged DNA in normal human fibroblasts.

In normal human cells the amount of excision of ultraviolet damage to DNA saturates at high doses. In these cells some chemicals mimic ultraviolet damage as far as their biological and repair characteristics are concerned. One of these chemicals is N-acetoxy-2-acetylaminofluorene. We determined whether the limited repair capacity for ultraviolet damage was affected by treatment with N-acetoxy-2-acetylaminofluorene. To measure repair we determined unscheduled DNA synthesis and the number of sites sensitive to an ultraviolet endonuclease in an assay using an extract of Micrococcus luteus. The nuclease does not act on DNA treated with the chemical. The amount of unscheduled DNA synthesis due to a combined chemical and ultraviolet treatment was the sum of those observed from the separate treatments, even at saturation doses. The combined treatment did not affect the removal of nuclease-sensitive sites. We conclude that there are different rate-limiting steps in excision repair of the ultraviolet and the chemical damage and suggest a model involving a complex of enzymes to explain the data.