P‐aminobenzoic Acid Can Sensitize the Formation of Pyrimidine Dimers in Dna: Direct Chemical Evidence

Thymine‐containing photoproducts with chromatographic properties similar to those of cyclobutyl pyrimidine dimers can be formed in [ 3 H]‐thymine‐labeled DNA in solution by 313 nm ultraviolet radiation in the presence of para‐aminobenzoic acid (PABA), a compound used in sunscreen preparations. In the absence of PABA, similar fluences of 313 nm radiation do not produce significant numbers of these photoproducts. The thymine‐containing photoproducts can be reversed by 254 nm radiation so that the tritium label migrates with the mobility of thymine monomer, a behavior characteristic of thymine‐containing cyclobutyl pyrimidine dimers. This result supports previous, but less direct, data from other laboratories indicating that PABA can sensitize dimer formation in the DNA of bacterial and mammalian cells.