A NEW CLASS OF DNA PHOTOLYASES PRESENT IN VARIOUS ORGANISMS INCLUDINGAPLACENTAL MAMMALS

DNA photolyase specifically repairs UV light-induced cyclobutane-type pyrimidine dimers in DNA through a light-dependent reaction mechanism. We have obtained photolyase genes from Drosophila melanogaster (fruit fly), Oryzias latipes (killifish) and the marsupial Potorous tridacty lis (rat kangaroo), the first photolyase gene cloned from a mammalian species. The deduced amino acid sequences of these higher eukaryote ge nes show only limited homology with microbial photolyase genes. Togeth er with the previously cloned Carassius auratus (goldfish) gene they f orm a separate group of photolyase genes. A new classification for pho tolyases comprising two distantly related groups is proposed. For func tional analysis P.tridactylis photolyase was expressed and purified as glutathione S-transferase fusion protein from Escherichia coli cells. The biologically active protein contained FAD as light-absorbing cofa ctor, a property in common with the microbial class photolyases. Furth ermore, we found in the archaebacterium Methanobacterium thermoautotro phicum a gene similar to the higher eukaryote photolyase genes, but we could not obtain evidence for the presence of a homologous gene in th e human genome. Our results suggest a divergence of photolyase genes i n early evolution.