Toxicity of L-Tryptophan photoproduct on recombinationless (rec) mutants of Salmonella typhimurium.

l-Tryptophan after exposure to black light becomes toxic for recombinationless (rec) mutants of Salmonella typhimurium. Fifty-six radiation-sensitive mutants were screened for sensitivity to the tryptophan photoproduct; the rec and exr (X-ray sensitive) mutants are sensitive, whereas the uvr, hcr, and wild-type strains are resistant. A number of catabolic products of tryptophan and compounds related to tryptophan were screened for toxicity to rec strain; these are nontoxic or far less toxic for rec strains than irradiated l-tryptophan. The toxic photoproduct is relatively stable to drying and basic hydrolysis at 90 C, indicating that it is a stable organic compound, eliminating peroxide as the toxic component. It was also observed that the toxic product is a photooxidation product since it is formed only when l-tryptophan is irradiated in the presence of air.