Tanning as part of the eukaryotic SOS response.

We have determined that DNA damage is at least one of the signals generated by ultraviolet radiation that stimulates pigmentation (tanning) in human skin. This photoprotective response is functionally similar to the SOS response described in bacteria. Here we present evidence that DNA damage stimulates pigmentation, at least in part, through up-regulation of tyrosinase mRNA and protein levels. Furthermore, this response can be induced in the absence of DNA damage by treatment of melanocytic cells and intact skin with small DNA fragments, particularly thymidine dinucleotides, pTpT. Topical application of these DNA fragments should provide a photoprotective tan to human skin without the harmful effects of ultraviolet radiation.