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Lethal Effects of Different LET Radiations
in Deinococcus radiodurans
Introduction
Ionizing radiation induces DNA double-strand breaks
(DSBs), which is a particularly serious form of DNA
damage and has an especially deleterious effect on cells.
The radiosensitivity of organisms varies extensively
depending on the species. Ion beams have a high linear
energy transfer (LET, keV/µm) and give DNA damage
containing DSBs locally (clustered damage) than gamma
rays do. In mammalian cells, plants and Escherichia coli,
the relative biological effectiveness (RBE) depends on LET
and exhibits the peak at a range of LET from about 100 to
200 keV/µm 1-3) .
While, Deinococcus radiodurans exhibits extraordinary
resistance to the lethal effects of ionizing and UV radiations,
as well as many other DNA damaging agents. This
resistance has been attributed to its highly proficient DNA
repair capacity 4) . The most noteworthy characteristic of D.
radiodurans is its capacity for repairing ionizing
radiation-induced DSBs. However, LET-dependent
biological effect in D. radiodurans is poorly understood.
In this study, we investigated lethal effects and relationship
between LET and RBE for different LET radiations.
Experimental procedures
D. radiodurans cells were cultivated at 30 °C in TGY
medium with agitation to early stationary phase. Cells
were harvested, washed and resuspended in 100 mM
Tris-HCl (pH 7.0) containing 2.5% glycerol (TG buffer).
Aliquots (1 mL) of the cell suspensions were adhered onto
cellulose membrane, frozen at – 80 °C and dried in vacuo.
The freeze-dried cells were irradiated with four kind of ion
beams ( 4 He 2+ [50 MeV; 19.4 keV/µm], 12 C 5+ [220 MeV;
121.8 keV/µm], 20 Ne 8+ [350 MeV; 441 keV/µm], 40 Ar 13+
[460 MeV; 1,650 keV/µm]) accelerated by AVF cyclotron,
or with 60 Co gamma rays (0.2 keV/µm) at Food Irradiation
Facility, JAEA. The irradiation dose ranged from 1 to
15 kGy. Irradiated cells were harvested, diluted
appropriately with the TG buffer, spread onto TGY agar,
and incubated at 30 ˚C for 3 days prior to the enumeration of
colonies. The RBE was calculated from the equation: RBE
= D10 of gamma rays (Gy) / D10 of ion beams (Gy), where
D 10 is the dose quantity necessary for decreasing the
survival fraction to 10%.
Results and discussion
To determine the cell condition for ion beam irradiation,
the sensitivities for gamma rays were compared between the
freeze-dried and wet cells. The freeze-dried cells exhibited
a higher resistance to gamma-irradiation than the wet cells
(Fig. 1), indicating that the effect of water radiolysis was
relieved under the freeze-dried condition. From this result,
the freeze-dried cells were used for ion beam irradiation.
As shown in Fig. 1, the radioresistances of freeze-dried
JAEA-Review 2010-065
K. Satoh, K. Tejima and I. Narumi
Radiation-Applied Biology Division, QuBS, JAEA
- 80 -
cells to He, C and Ne ion beams were almost equal to that of
the gamma-irradiated cells. On the other hand, the
high-LET Ar ion-irradiated cells exhibited a much higher
resistance than those of other radiations at high doses (10 to
15 kGy). This higher resistance to Ar ion beams was
attributed to overkill effect. Unlike other organisms, D.
radiodurans did not show the dependence of LET on RBE at
a range of LET from about 0.2 to 441 keV/µm (Fig. 2).
These results suggest that D. radiodurans could repair the
clustered damages effectively.
References
1) N. Hamada et al., Radiat. Res. 166 (2006) 24.
2) Y. Hase et al., Int. J. Radiat. Biol. 78 (2002) 799.
3) M. Imamura et al., J. Gen. Appl. Microbiol. 43 (1997)
175.
4) I. Narumi, Trends Microbiol. 11 (2003) 422.
Fig. 1 Survival curves to different LET radiations.
Symbols: closed circles, gamma rays (wet cells);
open circles, gamma rays (freeze-dried cells); open
triangles, 4 He 2+ ; open lozenges, 12 C 5+ ; open squares,
20 Ne 8+ ; closed squares, 40 Ar 13+ .
Fig. 2 The relationship between LET and RBE in D.
radiodurans.