Redox-active enzyme cofactors derived from ribonucleotides have been called "fossils of the RNA world," suggesting that early catalysts employed modified nucleo-bases to facilitate redox chemistry in primitive metabolism. Here, we show that the common oxidative damage product 8-oxo-7,8-dihydroguanine (OG), when incorporated into a DNA or RNA strand in proximity to a cyclobutane pyrimidine dimer, can mimic the function of a flavin in photorepair. The OG nucleotide acts catalytically in a mechanism consistent with that of photolyase in which the photo excited state of the purine donates an electron to a pyrimidine dimer to initiate bond cleavage; subsequent back electron transfer regenerates OG. This unusual example of one form of DNA damage, oxidation, functioning to repair another, photo-dimerization, may provide insight into the origins of prebiotic redox processes.