Abstract
We have shown that ultraviolet irradiation increases the non-conservative incorporation of dTTP into DNA using cells made permeable to ATP and deoxynucleoside triphosphates by freezing. The role of DNA polymerase I in this incorporation has been studied using N- ethylmaleimide as a selective inhibitor of DNA polymerases II and III. Ultraviolet stimulation of dTTP incorporation requires ATP and is sensitive to N- ethylmaleimide . However, the inhibitor can no longer block the increase in dTTP incorporation if the cells are incubated for 30 min after irradiation before DNA synthesis is allowed. Since N- ethylmaleimide inhibits excision of pyrimidine dimers from the DNA of irradiated cells, it may be that an early step in dimer excision is required for an increase in dTTP incorporation. Because mutants of Bacillus subtilis lacking DNA polymerase I cannot carry out a high rate of dTTP incorporation when assayed in this manner, and because DNA polymerase I is not sensitive to N- ethylmaleimide while polymerases II and III are sensitive, we conclude that DNA polymerase I very likely catalyzes essentially all of the non-conservative DNA synthesis in both unirradiated and irradiated frozen cells.
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