No abstract

No abstract

Abstract— Escherichia coli photoreactivating enzyme (PRE) has been purified in large amounts from an E. coli strain lysogenic for a defective Λ bacteriophage carrying the phr gene. The resulting enzyme has a pH optimum of 7.2 and an ionic strength optimum of 0.18. It consists of an apoprotein and cofactor, both of which are necessary for catalytic activity. The apoprotein has a monomer molecular weight of35,200 and shows stable aggregates under denaturing conditions. The amino acid analysis of the E. coli enzyme is very similar to that of the photoreactivating enzyme from orchid seedlings (Cattelya aurantiuca). Both have arginine at the amino terminus. The cofactor, like the holoenzyme, shows absorption, magnetic circular dichroism, and emission properties indicative of an adenine moiety. Although the isolated enzyme has an action spectrum which peaks at about 360 nm, neither the cofactor, apoenzyme nor holoenzyme shows any detectable absorption between 300 and 400 nm.

No abstract

Photoproducts formed in the DNA of human cells irradiated with ultraviolet light (uv) were identified as cyclobuytl pyrimidine dimers by their chromatographic mobility, reversibility to monomers upon short wavelength uv irradiation, and comparison of the kinetics of this monomerization with that of authentic cis-syn thymine-thymine dimers prepared by irradiation of thymine in ice. The level of cellular photoreactivation of these dimers reflects the level of photoreactivating enzyme measured in cell extracts. Action spectra for cellular dimer photoreactivation in the xeroderma pigmentosum line XP12BE agree in range (300 nm to at least 577 nm) and maximum (near 400 nm) with that for photoreactivation by purified human photoreactivating enzyme. Normal human cells can also photoreactivate dimers in their DNA. The action spectrum for the cellular monomerization of dimers is similar to that for photoreactivation by the photoreactivating enzyme in extracts of normal human fibroblasts.