Reassembly of shattered chromosomes in Deinococcus radiodurans

Abstract

Dehydration or desiccation is one of the most frequent and severe challenges to living cells. The bacterium Deinococcus radiodurans is the best known extremophile among the few organisms that can survive extremely high exposures to desiccation and ionizing radiation, which shatter its genome into hundreds of short DNA fragments. Remarkably, these fragments are readily reassembled into a functional 3.28-megabase genome. Here we describe the relevant two-stage DNA repair process, which involves a previously unknown molecular mechanism for fragment reassembly called 'extended synthesis-dependent strand annealing' (ESDSA), followed and completed by crossovers. At least two genome copies and random DNA breakage are requirements for effective ESDSA. In ESDSA, chromosomal fragments with overlapping homologies are used both as primers and as templates for massive synthesis of complementary single strands, as occurs in a single-round multiplex polymerase chain reaction. This synthesis depends on DNA polymerase I and incorporates more nucleotides than does normal replication in intact cells. Newly synthesized complementary single-stranded extensions become 'sticky ends' that anneal with high precision, joining together contiguous DNA fragments into long, linear, double-stranded intermediates. These intermediates require RecA-dependent crossovers to mature into circular chromosomes that comprise double-stranded patchworks of numerous DNA blocks synthesized before radiation, connected by DNA blocks synthesized after radiation.

Double-strand breaks associated with repetitive DNA can reshape the genome

“…The efficient repair of G 2 /M-induced DSBs displayed in yeast resembles the extraordinary HR properties of the radioresistant bacterium Deinococcus radiodurans (27). Both organisms have similar amounts of repetitive DNA [3.8% in D. radiodurans (28)].…”

Endogenous nitric oxide regulates the recovery of the radiation-resistant bacterium Deinococcus radiodurans from exposure to UV light

“…Deinococcus radiodurans (Dr) is especially adapted to survive UV radiation, ionizing radiation, desiccation, and oxidative damage (23,24). Dr adaptation involves multiple protective mechanisms, including efficient homologous recombination among its 8-10 genome copies, a tight nucleoid organization, and unusually high intracellular Mn/Fe ratio, which can support/ participate in protection against oxidative damage (23)(24)(25)(26). Nevertheless, most of the implicated genes are similar to those found in other organisms, and the repair mechanisms themselves are not unusual (23)(24)(25)(26).…”

Deinococcus radiodurans« … Si tu peux voir détruit l’ouvrage de ta vie… »

“…Car, contrairement à ce que pourrait laisser supposer son nom, les irradiations provoquent la même fragmentation de l'ADN chez cette bactérie radiodurans que chez les autres. [5]. Les techniques utilisées ont comporté, entre autres, l'électrophorèse en champ pulsé pour mesurer la taille des fragments au fur et à mesure de leur fusion, l'adjonction de thymidine tritiée et de BrdU pour repérer les brins synthétisés par densité et par photolyse au niveau moléculaire, et l'utilisation de la microscopie en fluorescence pour les observer à l'échelon cellulaire.…”