The Photochemical Addition of Amino Acids and Proteins to Nucleic Acids

Abstract

When bacterial cells are irradiated with ultraviolet light, DNA and protein become photochemically cross-linked. This photochemical reaction appears to be the major lethal lesion in irradiated bacteria under certain experimental conditions. We have turned to in vitro model systems in order to gain some insight into the mechanism(s) by which DNA and protein are photochemically cross-linked in vivo. A survey was performed of the abilly of the 22 common amino acids to add photochemically (254 nm) to 14C-uracil. The 11 reactive amino acids were glycine, serine, phenylalanine, tyrosine, tryptophan, cystine, cysteine, methionine, histidine, arginine and lysine. The most reactive amino acids were phenylalanine, tyrosine and cysteine. We have isolated the cysteine adduct and have shown it to be 5-S-cysteine-6-hydrouracil. The analogous thymine adduct has also been isolated and its tentative structure is 5-S-cysteine-6-hydrothymine. We have studied the kinetics of the photochemical addition of 35S-cysteine to various synthetic and natural polynucleotides. Preliminary data on the tyrosine adducts to uracil are reported.