Dehydration or desiccation is one of the most frequent and severe challenges to living cells1. 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 fragments2,3,4,5. 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.
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We thank J. Battista for initial help with Deinococcus and for the polA mutant; M. Meselson, B. Wagner, A. Stark, D. Zahradka and members of the M.R. lab for reading this manuscript and/or providing advice; M. Blazevic for technical assistance on γ-irradiation; and Mikula Radman for illustrations. K.Z. held a fellowship from the Necker Institute during work in M.R.'s laboratory. D.S. holds a Boehringer Ingelheim Foundation predoctoral fellowship and A.B.L. is a Marie Curie fellow. The laboratory in the Université de Paris-Descartes was funded by INSERM and the Necker Institute (Mixis/PLIVA contract); that in the Institut de Génétique et Microbiologie by EDF-France and CNRS (GEOMEX); that in the Institut Curie by EDF-France; and that in the Ruder Boskovic Institute by the Croatian Ministry of Science, Education and Sports. Author Contributions Experiments in Figs 1 and 2 and Supplementary Fig. 4 were carried out by K.Z.; those in Fig. 3 by D.S. A.B.L. provided expertise in microscopy; and the global experimental design was by M.R. The results of Fig. 1b, f, were obtained in, and with the scientific and material support of, the Institut de Génétique et Microbiologie; those of Fig. 1a, e, in the Institut Curie; those of refs Figs 1c, d, and Supplementary Fig. 4 in the Ruder Boskovic Institute, and those of Fig. 3 in the Université de Paris-Descartes.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Analysis of repaired D. radiodurans DNA by 5-BrdU density labelling. (PPT 55 kb)
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Zahradka, K., Slade, D., Bailone, A. et al. Reassembly of shattered chromosomes in Deinococcus radiodurans. Nature 443, 569–573 (2006). https://doi.org/10.1038/nature05160
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