Irradiation of the template with high-intensity (pulse-laser) ultraviolet light results in DNA-polymerase termination events at deoxyguanosine residues.

During primer elongation by Escherichia coli DNA-polymerase I large fragments on the template were irradiated with UV laser pulses at an intensity greater than or equal to 10(10) W/m2. In addition to the termination events at photoproducts typical of low-intensity UV irradiation, termination is observed before deoxyguanosine residues. The effect of the UV light intensity on the ratio of termination efficiencies before dPy and dG suggests that the termination of polymerization before deoxyguanosine residues results from the formation of photoproducts yielded by two-quantum reactions. The results obtained herein, together with data published previously, imply that photomodification of dG residues is the major two-quantum reaction under the action of high-intensity UV radiation on DNA.