Analysis of photoenzymatic repair of UV lesions in DNA by single light flashes. XII. Evidence for enhanced photolysis enzyme-substrate complexes by a 2-photon reaction.

Yeast photoreactivating enzyme (PRE), preilluminated with wavelengths ranging from the near-UV to the red spectral region, forms with 254 nm-irradiated transforming DNA of Haemophilus influenzae enzyme-substrate complexes that are more efficiently photorepaired than complexes formed from non-preilluminated PRE. The action spectrum for this "preillumination effect", previously shown to have a maximum in the near-UV region, has another maximum near 577 nm. In complexes formed from non-preilluminated PRE the repair probability per incident photon is only about 25% of that in complexes formed from preilluminated PRE, if low-intensity photoreactivating light is applied continuously or as a sequence of flashes. However, photoreactivating light in the form of a single, high-intensity flash of 1 msec duration raises the repair probability to greater than 50%. Two light flashes, discharged with a delay of slightly more than a millisecond, may already achieve less photorepair than the same energy given as a single flash. These results are explained by the assumption that the great majority of PRE molecules in a non-preilluminated preparation have reduced activity (of the order of 1/4 of maximal activity). These less reactive molecules form enzyme-substrate complexes ("non-activated complexes") in which the repair probability per incident photon is considerably increased if 2 or more photons are absorbed within a time period of the order of milliseconds. This phenomenon, tentatively termed "2-photon photolysis" does not occur in "activated complexes" (i.e. those formed form preilluminated enzyme). The data are compatible with suggestion that the first absorption leads to a metastable excited state of the complex, during which the repair probability is increased by absorption of another photon. The generally observed heterogeneity of the photolytic response of enzyme-substrate complexes can be partly explained by heterogeneity of PRE molecules regarding their activity. In particular, uncontrolled exposure of enzyme to almost any kind of room light before its experimental use can enhance the heterogeneity.