For most living organisms, a few DNA double-strand breaks generated by ionizing radiation are lethal. However, bacteria belonging to the genus Deinococcus are known for being very tolerant to high doses of ionizing radiation. This tolerance is related to the capacity to efficiently repair massive DNA lesions, including hundreds of double-strand breaks. It should be noted that natural sources of ionizing radiation on Earth exist at levels much lower than those used to generate large numbers of double-strand breaks, suggesting that radiation tolerance is a consequence of the bacterial response to natural non-radioactive DNA damaging conditions such as desiccation. Indeed, Deinococcus bacteria are not only extremely resistant to gamma radiation but also to desiccation and UV radiation.
Of the Deinococcus species, only Deinococcus radiodurans strain R1 has been extensively studied. High doses of gamma radiation, as well as prolonged periods of desiccation, generate numerous double-strand breaks in the genome of D. radiodurans. It has been shown that D. radiodurans is able to repair the radiation-induced breaks within a few hours with cells grown and recovered in rich growth medium. The genome of D. radiodurans R1 has been completely sequenced and analyzed (Makarova et al., 2001, Microbiol Mol Biol Rev 65: 44), and the effects of exposure of the cells to gamma radiation on the transcriptome and proteome have been determined, demonstrating modulation of expression of numerous genes. Based on what is known at present, three hypotheses to explain its radiation-tolerance can be proposed:
Despite these hypotheses, the genome repair mechanisms of D. radiodurans, and thus its extreme radiation-tolerance, are still largely unclear (Cox & Battista, 2005, Nat Rev Microbiol 3: 882).
A comparative genomic analysis of D. radiodurans with a related species will likely improve our comprehension of the mechanisms involved in the extreme radiation tolerance in Deinococcus. For this analysis, Deinococcus deserti strain VCD115 has been selected. D. deserti, recently isolated from the Sahara desert, does not grow in rich media in contrast to D. radiodurans (de Groot et al., 2005, Int J Syst Evol Microbiol 55 : 2441). D. deserti cells are non-motile rods, whereas those of D. radiodurans are spherical. In contrast to the red or pink colonies of other Deinococcus species, those of D. deserti are whitish. D. deserti stains Gram-negative, whereas D. radiodurans stains Gram-positive. The genome size of D deserti VCD115 has been estimated at 3.8 Mb, whereas the sequenced D. radiodurans genome is 3.2 Mb. We have shown that D. deserti VCD115 is very tolerant to gamma and UV radiation, as well as to desiccation (de Groot et al., submitted). Exposure of D. deserti to a high dosis of gamma radiation results in the fragmentation of its genome (double-strand breaks), but D. deserti has the capacity to repair it within a few hours when incubated in poor growth medium.
For a better understanding of the extreme radiation-tolerance in Deinococcus, D. deserti will be compared to D. radiodurans, starting with the genomic approach, then with other global (transcriptome and proteome) and targeted approaches (genetics, biochemical, structural). For this comparative study, sequencing of the genome of D. deserti VCD115 is in progress. With the D. deserti genome sequence, it will be possible to determine:
The comparative and functional analysis should allow us to improve our comprehension of the mechanisms implicated in DNA repair and the extraordinary resistance to gamma and UV radiation and to desiccation.