Analysis of photoernzymatic repair of UV lesions in DNA by single light flashes. XI. Light-induced activation of the yeast photoreactivating enzyme

Photoreactivating enzyme (PRE) from yeast (as semi-crude extract, or in highly purified form) shows increased activity if its is illuminated with near UV or short wavelength visible light prior to its use for photoenzymatic repair of UV-induced pyrimidine dimers in transforming DNA in vitro. This effect results from an alternation in PRE molecules changing those with low activity in the light-dependent step of the reaction to a higher activity. Light-induced activation of PRE preparations is slowly lost by dark storage for several hours to 1 day (faster at 23°C than at 5°C), but can be recovered repeatedly by renewed preillumination. The action spectrum for these preillumination effects generally resembles that for the photoenzymatic repair reaction itself, having its maximum in the same 355–385 nm region as the latter, but light of somewhat longer wavelengths (546 nm) is still effective. Preilluminated PRE is also more stable to thermal inactivation (65°C) than untreated enzyme.