INTRODUCTION

Cis,syn-cyclobutane pyrimidine dimers are one of the major photoproducts produced in DNA by radiation of UV light.  Left uncorrected, these dimers will inhibit cells from being able to replicate or transcribe DNA.  This inhibition may cause the DNA to exhibit cellular mutations and/or cancer which can eventually cause cell death.  In order to prevent this occurrence, cells will proceed through excision repair or photoreactivation.  Excision repair is often only used if photoreactivation cannot be done.  Photoreactivation prevents the effects of UV light by concurrent or subsequent exposure to visible light (Sancar, 1994).  A photoreactive enzyme known as DNA photolyase must be present in the cell to catalyze this reaction.  However, even in the presence of photolyase, this process can only occur if there is an adequate amount of blue light photons present.  If the proper conditions are not available, the enzyme will proceed to aid the cell in correcting the dimer by stimulating excision repair.    

DNA photolyase binds directly to the dimer and upon excitation with blue light it splits the dimer to restore proper structure in the DNA. This occurs with comparable affinities in both the double and single stranded DNA (Park,etal.,1995).   However, the affinity of this reaction is affected by the base composition of the dimer.   The affinity of the enzyme is the highest when its substrate is a thymidine dimer (Sancar, 1994).

There is a wide distribution of DNA Photolyase in nature but  it is absent in many species in an unpredictable manner(Ozer, etal.,1995).  It has been found in bacteria, archaebacteria, and the internal organs of eukaryotes such as gold fish, rattlesnakes, and marsupials (Park, etal.,1995).  It is believed that humans and other placental mammals lack this enzyme.

The enzyme has been divided into the deazaflavin and folate classes.   Both classes posses the catalytic cofactor flavin adenine dinucleotide (FADH).  The second cofactor, which acts as an antenna in the light harvesting reaction, determines the classification of the enzyme. 8-hydroxy-5-deazariboflavin (8-HDF) is present in the deazaflavin class while methenyltetrahydrofolate (MTHF) is present in the folate class.  

Some specific species belonging to the folate class are Escherichia coli, Saccharomyces cerevisiae, Neurospora crassa, and Bacillus firmus.   Species such as Anacystis nidulans, Streptomyces, griseus, Scenedesmus acutus, and Methanobacterium thermoautotrophicum belong to the deazaflavin class(Sancar, 1994).

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Structure