DNA-specific damage results from exposure of DNA to ultraviolet light. The major photoproduct is due to dimerization of adjacent nucleic acid bases, the cyclobutane pyrimidine dimer (CPD). Many organisms use a very efficient DNA repair enzyme, DNA photolyase, to reverse this DNA-specific damage. DNA photolyase belongs to the class of blue-light photoreceptors and reverses the mutagenic CPD damage via a light-driven, electron transfer mechanism. When the enzyme is one electron oxidized from its active form, it can undergo a photoreduction reaction back to the active form which presumably involves electron transfer through a chain of tryptophan amino acids which are highly conserved within the class of blue-light photoreceptors. In this project, state-of-the-art ultrafast laser spectroscopy will be used to determine the structural changes in the photolyase active site after absorption of light to understand the primary processes of DNA photorepair and how the presence of damaged DNA modifies these processes on a molecular level. The same methodology will be applied to characterize the individual reaction intermediates that are involved in the tryptophan electron transfer process. The approach uses time-resolved resonance Raman spectroscopy with picosecond time-resolution (one millionth of a millionth of a second), biosynthetic isotopic labeling of the flavin molecule and tryptophan residues, and high-level calculations to interpret the results. The project also involves the training of graduate students and postdoctoral research associates in the application of ultrafast laser spectroscopy, along with training of undergraduate students in the basic biophysics, biochemistry and spectroscopy methods used.
