Fig. 6-9 DNA damage induced by ionizing and non-ionizing radiation Left: Ionization of water molecules close to the DNA by ionizing radiation generates active hydroxyl radicals. The hydroxyl ions oxygenize a nucleotide base within the DNA (indirect effect of radiation). In the case of a direct hit by ionizing radiation on DNA, various modifications are found to occur in the DNA (direct effect of radiation). Right: Ultraviolet rays that are non-ionizing radiation induce dimer formation in pyrimidine. Pyrimidine dimerization is not induced by ionizing radiation.

Fig. 6-10 Survival curve of D. radiodurans following ultraviolet irradiation Whereas a mutant deficient in the prokaryote-type nucleotide excision repair gene uvrA exhibits ultraviolet resistance, as does a wild-type strain, a double deficient strain that lacks the functions of both uvrA and a eukaryote-type repair gene uvde is extremely sensitive to ultraviolet rays.

Fig. 6-11 The amino acid sequence of a DNA repair enzyme encoded by the D. radiodurans uvde gene In ultraviolet ray sensitive mutants, arginine (Arg) at amino acid position 133 becomes mutated to glutamine (Gln), or glutamic acid (Glu) at amino acid position 311 becomes mutated to lysine (Lys). These mutations result in malfunction of the DNA repair enzyme.

In recent years, increase of harmful ultraviolet rays caused by depletion of the ozone layer has come under close scrutiny. It is estimated that O2 began to be released into the atmosphere by photosynthetic microorganisms 1.9 billion years ago, culminating in accumulation of ozone surrounding the earth, thus protecting it from cosmic radiation and solar ultraviolet waves. A stable stratospheric ozone layer was formed about 500 million years ago, when O2 concentration in the atmosphere reached 20%. After formation of a stable ozone layer, it was possible for living organisms to survive on the ground. The type of DNA damage induced by ultraviolet radiation is different from that induced by ionizing radiation such as gamma-rays (Fig. 6-9). Deinococcus radiodurans, which is known as a radiation-resistant bacterium, also exhibits very high resistance to ultraviolet rays (Fig. 6-10). This bacterium is considered to be a survivor which acquired resistance not only to ionizing radiation but also to ultraviolet radiation in the course of evolution after its appearance about 2 billion years ago. DNA damaged by ultraviolet radiation can mainly be repaired via photoreactivation and nucleotide excision repair processes. Despite of the lack of a photoreactivation repair pathway in D. radiodurans, mutants deficient in a prokaryote-type nucleotide excision repair are known to show resistance to ultraviolet rays. The reason why repair-deficient mutants do not exhibit ultraviolet sensitivity has been a longstanding mystery. This mystery has been solved by our discovery that D. radiodurans possesses a eukaryote-type nucleotide excision repair gene, as do Neurospora and budding yeast, as well as a prokaryote-type nucleotide excision repair (Fig. 6-11). In other words, D. radiodurans developed two independent repair pathways (prokaryote-type and eukaryote-type) in the evolutional process, and the function of these two pathways renders this bacterium extraordinarily resistant to ultraviolet rays.

Reference S. Kitayama et al., Cloning of Structural Gene of Deinococcus radiodurans UV-Endonuclease beta, Biosci. Biotechnol. Biochem., 67(3), 613 (2003).

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