Action of T4 endonuclease V on DNA containing various photoproducts

The action of T4 endonuclease V on DNA containing various photoproducts was investigated. (1) The enzyme introduced strand breaks in DNA from ultraviolet-irradiated vegetative cells of Bacillus subtilis but not in DNA from irradiated spores of the same organism. DNA irradiated with long wavelength (360 nm peak) ultraviolet light in the presence of 4,5',8-trimethylpsoralen was not attacked by the enzyme. These results indicate that 5-thyminyl 5,6-dihydrothymine (spore photoproduct) and psoralen mediated cross-links in DNA are not recognized by T4 endonuclease V. (2) DNA of phage PBS1, containing uracil in place of thymine, and DNA of phage SPO1, containing hydroxymethyluracil in place of thymine, were fragmented by the enzyme when the DNA's had been irradiated with ultraviolet light. T4 endonuclease V seems to act on DNA with pyrimidine dimers whether the dimers contain thymine residues or not.