Monomerization of pyrimidine dimers in DNA by tryptophan-containing peptides: wavelength dependence.

Tryptophan-containing peptides and proteins can sensitize the monomerization of pyrimidine dimers in ultraviolet-irradiated DNA; photoreactivating enzymes catalyze the light-induced monomerization of pyrimidine dimers in DNA. It has recently been proposed that a variety of tryptophan-containing proteins and peptides might be confused with true photoreactivating enzymes both in vivo and in vitro. We have thus characterized the wavelengths required for the tryptophan-sensitized dimer monomerization to determine if this process is distinguishable from true enzymatic photoreactivation. We find that 313-nm radiation can monomerize pyrimidine dimers in DNA in the presence of the peptide lysyl-tryptophyl-lysine; however, 334- 365-, and 405-nm radiation are ineffective for fluences up to 1 MJ/m/sup 2/. In contrast, each of these wavelengths is capable of monomerizing dimers in the presence of photoreactivating enzymes. Indeed, 334 and 365 nm are always more effective than 313-nm radiation in the case of true enzymatic photoreactivation. The inability of wavelengths other than those near 300 nm to drive the tryptophan-mediated reaction efficiently is consistent with recently reported spectroscopic experiments. The extreme differences in the wavelength specificities for true enzymatic photoreactivation and tryptophan-sensitized monomerization mean that it is easy to differentiate experimentally between the two phenomena. Consideration of the spectral distributions of conventional sourcesmore » of photoreactivating light indicate that it is extremely unlikely that any of them could contain significant intensities of the wavelengths required for efficient tryptophan-sensitized monomerization of pyrimidine dimers. We thus conclude that tryptophan-sensitized monomerization cannot account for the disappearance of pyrimidine dimers from DNA in cells or cell extracts exposed to photoreactivating light.« less