Action mechanism of Escherichia coli DNA photolyase. III. Photolysis of the enzyme-substrate complex and the absolute action spectrum.

The absolute action spectrum of Escherichia coli DNA photolyase was determined in vitro. In vivo the photoreactivation cross-section (epsilon phi) is 2.4 X 10(4) M-1 cm-1 suggesting that the quantum yield (phi) is about 1.0 if one assumes that the enzyme has the same spectral properties (e.g. epsilon 384 = 1.8 X 10(4) M-1 cm-1) in vivo as those of the enzyme purified to homogeneity. The relative action spectrum of the pure enzyme (blue enzyme that contains FAD neutral semiquinone radical) agrees with the relative action spectrum for photoreactivation of E. coli, having lambda max = 384 nm. However, the absolute action spectrum of the blue enzyme yields a photoreactivation cross-section (epsilon phi = 1.2 X 10(3) at 384 nm) that is 20-fold lower than the in vivo values indicative of an apparent lower quantum yield (phi approximately equal to 0.07) in vitro. Reducing the enzyme with dithionite results in reduction of the flavin semiquinone and a concomitant 12-15-fold increase in the quantum yield. These results suggest that the flavin cofactor of the enzyme is fully reduced in vivo and that, upon absorption of a single photon in the 300-500 nm range, the photolyase chromophore (which consists of reduced FAD plus the second chromophore) donates an electron to the pyrimidine dimer causing its reversal to two pyrimidines. The reduced chromophore is regenerated at the end of the photochemical step thus enabling the enzyme to act catalytically.+