Photoreactivation Repair

Introduction image of PR

Photoreactivation repair is the DNA repair system, which corrects the damage (caused by UV-C and UV-B light radiations) with the aid of photolyase enzyme under the exposure to the visible light. The first evidence for the Photoreactivation was given by the two scientists, namely Hausser and Von Oehmcke, in the year 1933. Hausser and Von Oehmcke reversed the darkened banana skin (caused by UV-exposure) by the use of mercury arc lines at 366, 405 and 435nm.

Thus, Photoreactivation repair is a unique method, which repairs the DNA damage caused by the light with the use of light itself. In simple term, it is a light-dependent process and also refers as “Light repair”.

Content: Photoreactivation Repair

  1. Definition of Photoreactivation Repair
  2. Mechanism of Photoreactivation
  3. Role of Photolyase
  4. Process

Definition of Photoreactivation Repair

Photoreactivation repair can define as the DNA repair system where the DNA damaged by exposure to the UV-light, are repaired by the re-exposure of the DNA to the visible light. This process also refers as “Direct reversal DNA repair mechanism” in which the effect of UV-radiation on the cells can reverse with the help of photolyase enzyme under the action of visible light.

photoreactivation
It recovers the DNA from the UV-C (180-290nm) and UV-B (290-320nm) damage by the treatment with the light of longer wavelength (320-500nm). The process of photoreactivation can recover the damage of both prokaryotic and eukaryotic DNA.

In the year 1949, Kelner demonstrated an experiment on photoreactivation by plotting a standard graph between a percentage of survived cells and UV dose. The E.coli cells, when exposed to UV-light, it has shown exponential death with a broad curve. But when the E.coli cells were treated with the visible light, the size of the slope reduced, as shown in the graph below:

graph showing effect of UV light on DNA

Mechanism of Photoreactivation

Photoreactivation process repairs the DNA damage caused by the UV-radiations. Under the exposure of UV-light, a DNA helix becomes distorted and forms a pyrimidine dimer (usually thymine= thymine dimer).

DNA dimer caused by UV

A dimer forms when the adjacent thymine bases attach covalently to their respective C-5 and C-6 atoms. A formation of T=T dimer cause helix distortion, which leads to cause an error in replication of DNA and finally cause mutation.

To recover the helix distortion, the covalent bond formed between the two adjacent thymine bases must be broke. An enzyme photolyase breaks off the covalent bond between the dimer.

Role of Photolyase

The enzyme photolyase consists of FAD (Flavin adenine dinucleotide) and MTHF (Methylene tetra hydro folate) cofactors.

mechanism of Photolyase
First, to repair the DNA damage, the MTHF absorbs visible light of higher wavelength (Mostly of 384nm). After absorbing the energy coming from the visible light, MTHF gets excited. The Excitement of MTHF molecules transduces photons or electrons to the FAD molecule.

A FAD molecule then accepts the electron and convert into FADH2. FADH2 (A high energy molecule) will bind with the pyrimidine dimer. After binding, it breaks off the double covalent bond between the cyclobutane thymine bases. Thus, a photolyase reverses the cyclobutane pyrimidine bases to the usual bases of the DNA, by the photochemical reactions.

Process

The process of photoreaction is a light-dependent repair. It uses an enzyme photolyase which activates, by getting energy from the visible light of longer wavelength (320-500nm). Photoreactivation is a direct reversal repair mechanism where the cyclobutane pyrimidine dimers forms as a result of UV-exposure, are reversed by the exposure to visible light.

It mainly reverses the dimeric product of UV-light into usual monomeric form. The process of Photoreactivation includes the following steps:

process of photoreactivation repair

Formation of Pyrimidine-dimer

The UV-light induces the formation of a covalent bond between the pyrimidine bases either thymine or cytosine. The two adjacent pyrimidine bases bind by the means of carbon-carbon double bond. A formation of carbon-carbon double bond between the pyrimidine bases forms a ‘Dimeric structure’ commonly refers to as ‘Cyclobutane pyrimidine dimers’.

Cyclobutane pyrimidine dimers (CBPBs) is a very common product forms after the UV-exposure. In addition to this, 6-4 photoproducts also produce as a result of UV-damage, where the two bases attach with a single carbon bond to the C-6 of one ring and C-4 of another ring. Thymine=Thymine dimer usually forms after the UV-exposure, and it appears like a bubble in the DNA strand.

Treatment with Photolyase under visible light

Under the visible light of longer wavelength, a photolyase enzyme becomes activated. A photolyase enzyme scans the DNA strand and recognizes the pyrimidine dimers. After recognition of dimeric forms in the DNA, it directly binds to it.

Breaking of T=T dimer

When a photolyase enzyme binds to the T=T dimer, it starts splitting the carbon-carbon double covalent bond between the cyclobutane rings. Thus, a photolyase enzyme converts the dimeric form of the pyrimidines bases into the monomeric forms.

Release of Photolyase

After breaking the double covalent bond between the consecutive thymine bases, a photolyase enzyme releases out from the DNA strand, and the DNA fixes.

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