Biological effects and repair of damage photoinduced by a derivative of psoralen substituted at the 3,4 reaction site: photoreactivity of this compound and lethal effect in yeast.

A newly synthesized linear psoralen derivative, 3-carbethoxypsoralen is shown to bind to yeast nucleic acids after 365 nm light treatment. As compared to 8-methoxypsoralen, a well-known bifunctional furocoumarin, 3-carbethoxypsoralen exhibits a high photoaffinity for DNA in vivo. Both compounds bind and photoreact more efficiently in vivo than in vitro. In contrast to 8-methoxypsoralen, 3-carbethoxypsoralen does not form cross-links in yeast DNA as demonstrated by heat denaturation-reassociation studies at least in the range of doses used. Thus 3-carbethoxypsoralen reacts as a monofunctional compound. Wild-type cells of Saccharomyces cerevisiae are 6 times more resistant to 3-carbethoxypsoralen than to 8-methoxypsoralen plus 365 nm light treatment in terms of lethal effect. In comparison to angelicin, another monofunctional (but angular) furocoumarin, 3-carbethoxypsoralen is more photoreactive. When the photoaffinity for DNA of 8-methoxypsoralen and 3-carbethoxypsoralen are considered in relation to photoinduced cell killing, it is clear that monoadducts are very efficiently repaired in wild-type cells. In contrast to the additivity obtained with 8-methoxypsoralen, a synergistic interaction of the two different repair pathways blocked by the rad2 and the rad9 mutation is observed after 3-carbethoxypsoralen plus 365 nm light. Dark holding experiments show that the excision repair function which is present in wild-type and rad9-4 cells is important for dark recovery.