Escherichia coli photolyase uses blue light to repair cyclobutane pyrimidine dimers which are formed upon irradiation of DNA with ultraviolet (UV) light. E. coli photolyase is a flavoenzyme which contains a flavin adenine dinucleotide (FAD) in its active site and a 5,10-methenyltetrahydrofolate (MTHF) as a light-harvesting pigment. In the isolated enzyme, the FAD cofactor is present as a stable neutral radical semiquinone (FADH • ). In this paper, we investigate the interaction between photolyase and UV-damaged DNA by using resonance Raman and UV-vis spectroscopy. Substrate binding results in intensity changes and frequency shifts of the FADH • vibrations and also induces electrochromic shifts of the FADH • electronic transitions because of the substrate electric dipole moment. The intensity changes in the resonance Raman spectra can be largely explained by changes in the Raman excitation profiles because of the electrochromic shift. The size of the electrochromic shift suggests that the substrate binding geometry is similar to that of oxidized FAD in reconstituted photolyase. The frequency changes are partially a manifestation of the vibrational Stark effect induced by the substrate electric dipole moment but also because of small perturbations of the hydrogen-bonding environment of FADH • upon substrate binding. Furthermore, differences in the resonance Raman spectra of MTHF-containing photolyase and of an MTHF-less mutant suggests that MTHF may play a structural role in stabilizing the active site of photolyase while comparison to other flavoproteins indicates that the FAD cofactor has a strong hydrogen-bonding protein environment. Finally, we show that the electrochromic shift can be used as a direct method to measure photolyase-substrate binding kinetics.

The molten globule, a conformational ensemble with significant secondary structure but only loosely packed tertiary structure, has been suggested to be a ubiquitous intermediate in protein folding. However, it is difficult to assess the tertiary packing of transiently populated species to evaluate this hypothesis. Escherichia coli RNase H is known to populate an intermediate before the ratelimiting barrier to folding that has long been thought to be a molten globule. We investigated this hypothesis by making mimics of the intermediate that are the ground-state conformation at equilibrium, using two approaches: a truncation to generate a fragment mimic of the intermediate, and selective destabilization of the native state using point mutations. Spectroscopic characterization and the response of the mimics to further mutation are consistent with studies on the transient kinetic intermediate, indicating that they model the early intermediate. Both mimics fold cooperatively and exhibit NMR spectra indicative of a closely packed conformation, in contrast to the hypothesis of molten tertiary packing. This result is important for understanding the nature of the subsequent rate-limiting barrier to folding and has implications for the assumption that many other proteins populate molten globule folding intermediates.