The fate of pyrimidine dimers in ultraviolet-irradiated Chlamydomonas

Abstract— We have developed a chromatographic technique for the separation of 32 P‐labeled pyrimidine nucleotide dimers of the form Py p Py from 32 P‐phosphate in enzymatic hydroly sates of ultraviolet‐irradiated DNA. Application of this technique to 32 P‐labeled Chlamydomonas reinhardii shows that ultraviolet irradiation of this organism induces pyrimidine dimers in both nuclear and chloroplast DNA. We have found no evidence that these dimers are excised from either DNA species after several hours incubation under non‐photoreactivating conditions. A function has been derived to permit the pyrimidine‐dimer content determined from radioactive‐thymine‐labeled cells to be conveniently compared to that obtained from 32 P‐phosphate‐labeled cells.