UV PHOTOBIOLOGY AND REPAIR MECHANISMS 



RICHARD B. SETLOW 



Biology Department 



Brookhaven National Laboratory 



Upton, NY 11973 



ABSTRACT: The ultraviolet component of sunlight is the most potent environmental 

 agent that alters the structures of macromolecules. It has played an important 

 role in evolution and is responsible for a wide variety of biological effects, 

 such as inhibition of macromolecular synthesis, mutation of cells, killing of 

 cells, as well as deleterious effects on proteins and membranes. The effects on 

 DNA are probably the most important not only because DNA contains the information 

 in cells necessary for transcription and translation, but because DNA is the 

 largest molecule in cells and it has a significant absorption coefficient in the 

 UV-B region. In this region, the sensitivity of DNA is at least 10-fold greater 

 than that of other cellular structures. 



All biological systems have developed a number of strategies for minimizing the 

 effects of solar UV. DNA repair mechanisms presumably arose from the 

 evolutionary pressure of ultraviolet radiation and ameliorate a large fraction of 

 the ultraviolet effects. Two well -studied strategies are enzymatic 

 photoreactivation (the direct reversal of UV-induced pyrimidine dimers in DNA) 

 and nucleotide excision (the removal of photo-products from DNA by a cut and 

 patch mechanism operating in the dark). Exposure to sunlight involves the 

 simultaneous application of UV and photoreactivating illumination. Examples of 

 the combined effect of this type of treatment will be given. 



An understanding of the effects of the range of wavelengths present in sunlight 

 on aquatic and terrestrial ecosystems requires a knowledge of these effects on 

 representative components of the systems and a basic understanding of the causes 

 for such effects. From a photobiological point of view, the quantitative answers 

 to the following four questions are essential: 



1. What are the dose-response relations for monochromatic wavelengths? 



2. Do low intensities for a long time give the same result as high intensities 

 for a short time? (Does reciprocity hold?) 



3. What is the relative effectiveness of different monochromatic wavelengths in 

 producing the observed effect (the action spectrum)? 



4. Is the sum of the effects of monochromatic wavelengths additive, 

 antagonistic, or synergistic? 



Examples of answers to these questions, and the interpretation of the answers, 

 will be given for some well studied simple bacterial systems. 



The data to be discussed are derived from the references that follow. 



This work was supported by the Office of Health and Environmental Research of the 



U.S. Department of Energy. 



