5.7 The Gene Required for DNA Repair in a Radiation Resistant Bacterium
– A Contradiction of the Published Theory


Fig. 5-14 Electron micrograph of radiation resistant bacterium D. radiodurans

D. radiodurans shows 100 and 1,000 fold higher radiation resistance than Escherichia coli and human cells, respectively. D. radiodurans DNA receives the same damage by irradiation as E. coli and human DNA, however, the damaged DNA is repaired in a few hours in D. radiodurans.


Fig. 5-15 Identification of DNA repair gene in D. radiodurans

The D. radiodurans genome size is 3.29 mega base pairs. The DNA region conferring radiation resistance was localized in a DNA fragment with a 4,402 base pair length. Furthermore, it was revealed that there was only one difference in the 2,877th nucleotide composition between the normal strain and radiation sensitive mutant. The D. radiodurans recA gene located in this region has an indispensable function in this extraordinary radiation resistance.


Fig. 5-16 Property of D. radiodurans DNA repair protein

The recA gene encodes a DNA repair protein RecA that completes a repair reaction in cells. By introducing the recA gene in E. coli, the property of D. radiodurans RecA protein was evaluated. It was revealed that the D. radiodurans RecA protein shared the functions of RecA homologs from other radiation sensitive organisms such as E. coli and human. The published theory of the distinctiveness of D. radiodurans RecA was therefore reversed by our experiment.



Deinococcus radiodurans is known as a representative of radiation resistant bacteria (Fig. 5-14). The radiation resistance of this bacterium is approximately 1,000 fold higher than that of human cells. Many research results have supported the possibility that D. radiodurans possesses extremely proficient mechanisms by which enormous amounts of DNA damage are repaired accurately. D. radiodurans has a recA gene that plays a central role in DNA recombination repair and is conserved in almost all organisms. From previous studies, it has been believed that the extraordinary radiation resistance is due to a distinct function of the D. radiodurans RecA protein produced from the recA gene. We performed several experiments to reconfirm this published theory.
A radiation sensitive mutant has been isolated from D. radiodurans. By analyzing its nucleotide sequence, we identified a mutation site in the radiation sensitive mutant (Fig. 5-15). The mutation site was located in a recA-coding region, indicating that the D. radiodurans recA gene has an indispensable function in extraordinary radiation resistance. However, from our experiments, it was revealed that the D. radiodurans RecA protein shared the functions of RecA homologs from other radiation sensitive organisms (Fig. 5-16). Our result reverses the published theory of the distinctiveness of D. radiodurans RecA. The result further indicates that the presence of the recA gene is necessary, but not enough to account for extraordinary radiation resistance, and strongly suggests that D. radiodurans has other important genes that may cooperate with the recA gene.


Reference
I. Narumi et al., Molecular Analysis of the Deinococcus radiodurans recA Locus and Identification of a Mutation Site in a DNA Repair-Deficient Mutant, rec30, Mutat. Res., 435(3), 233 (1999).

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