Photochemical properties of Escherichia coli DNA photolyase: selective photodecomposition of the second chromophore.

Escherichia coli DNA photolyase contains a stable flavin radical and a second chromophore (SC) of unknown structure. The effects of flash (both conventional and laser) excitation of either the radical alone or both the radical and the second chromophore have been investigated by variation of the excitation wavelengths. Radical excitation leads to an electron abstraction by the lowest excited doublet state of the radical from an amino acid residue, probably a cysteine or tyrosine. On a longer time scale, a back-reaction occurs that can be prevented by the presence of certain electron donors, e.g., thiols, NADH, or tyrosine, but not pyrimidine dimers. Excitation of the second chromophore leads to electronic energy transfer from second chromophore excited states to the ground-state flavin radical doublet state, thus increasing the population of the lowest excited doublet state. Repetitive excitation of the enzyme with white light leads to photodecomposition of the second chromophore but not of the flavin adenine dinucleotide cofactor. Enzyme with photodecomposed SC retains full activity.