Base-specific damage induced by 4-thiouridine photosensitization with 334-nm radiation in M13 phage DNA

Site-specific DNA damage caused by 334-nm radiation in the presence of the rare Escherichia coli base 4-thiouridine was investigated in vitro by detecting the sites of the termination of DNA synthesis with irradiated M13 phage DNA used as a template. Single-strand breakage was also examined. The results indicate that 334-nm radiation at very low fluences in the presence of 4-thiouridine induces termination of strand synthesis at thymine base sites and at the base immediately prior to thymine. Termination at these sites was diminished by treatment with hot piperidine. Strand cleavage by piperidine treatments was observed preferentially at the guanine site, but only after irradiation at much larger fluences. It is hypothesized that at low fluences 4-thiouridine forms photoadducts with thymine that block DNA synthesis, while at high fluences the guanine site is damaged via oxygen species. (author)