The goal is to understand molecular mechanisms responsible for the robustness of bacteria facing extremely unfavourable life conditions. In particular, the aspects of protection from macromolecular damage and repair of inflicted damage to DNA, RNA and proteins will be explored. The organism of initial choice is the bacterium Deinococcus radiodurans, highly resistant to radiation and desiccation.

Deinococcus radiodurans, an extremophile bacterium, sustains extreme conditions of life, such as excessive desiccation and exposure to high doses of ionizing radiation. Both desiccation and radiation cause DNA double-strand breaks, the most severe form of genomic damage. Whereas most vegetative prokaryotic and eukaryotic cells can repair less than a dozen simultaneous double-strand DNA breaks, D. radiodurans survives extreme desiccation and ionizing radiation breaking its genome into several hundred fragments. Remarkably, in just couple of hours, these fragments are reassembled into functional chromosomes due to an efficient and precise DNA repair process. However, the molecular mechanism of this repair remained a mystery for 50 years.

We have recently found that genome reconstitution in D. radiodurans following gamma irradiation takes place as a two-stage process, which involves a novel mechanism called “extended synthesis-dependent strand annealing” (ESDSA), followed and completed by homologous recombination (Zahradka et al. 2006, Nature 443, 569-573). In ESDSA, chromosomal fragments produced by radiation are used both as primers and templates for a massive synthesis of long single-strand extensions. This synthesis depends on DNA polymerase I and incorporates more nucleotides than does normal replication in intact cells. Newly synthesized single-strand extensions become “sticky ends” that anneal with high precision, joining together contiguous DNA fragments into long linear intermediates. These intermediates are finally matured by RecA-mediated crossovers into functional circular chromosomes that comprise double-stranded patchworks of numerous DNA blocks synthesized before radiation, connected by DNA blocks synthesized after radiation.

Future prospects

The mechanism of desiccation and radiation resistance of D. radiodurans will be studied from different aspects. A part of our research will be focused on identifying novel genes, enzymes and processes that contribute to the efficiency and fidelity of DNA repair in this bacterium. Also, an extensive study of the robustness of deinococcal proteins by combined biochemical, biophysical and computational approaches will be undertaken.