ENERGY-TRANSFER (DEAZAFLAVIN -] FADH2) AND ELECTRON-TRANSFER (FADH2 -] T-LESS-THAN-GREATER-THAN) KINETICS IN ANACYSTIS-NIDULANS PHOTOLYASE

DNA photolyases catalyze the photocycloreversion of cyclobutane pyrimidine dimers. The enzyme from the cyanobacterium Anacystis nidulans contains two chromophores, 1,5-dihydroflavin adenine dinucleotide (FADH2) and 7,8-didemethyl-8-hydroxy-5-deazariboflavin (8-HDF). The photophysical/photochemical reactions leading to DNA repair were investigated by using time-resolved and steady-state fluorescence spectroscopy. It was found that the excited singlet state of 8-HDF transfers energy to FADH2 at a rate of 1.9 x 10(10) s-1 and a quantum yield of 0.98. Using the Forster equation for long-range energy transfer and assuming random orientations of the donor and acceptor the interchromophore distance was calculated to be 15 angstrom. The excited singlet FADH2 which forms either by energy transfer from 8-HDF or by direct absorption of a photon has a lifetime of 1.8 ns in the absence of substrate and 0.14 ns in the presence of the photodimer indicating electron transfer from the FADH2 excited singlet state to the dimer at a rate of 6.5 X 10(9) s-1 and quantum efficiency of 92%.