A MODEL FOR REPAIR OF RADIATION-INDUCED DNA DOUBLE-STRAND BREAKS IN THE EXTREME RADIOPHILE DEINOCOCCUS RADIODURANS

The bacterium Deinococcus (formerly Micrococcus) radiodurans and other members of the eubacterial family Deinococaceae are extremely resista nt to ionizing radiation and many other agents that damage DNA. Statio nary phase D. radiodurans exposed to 1.0-1.5 Mrad gamma-irradiation su stains >120 DNA double-strand breaks (dsbs) per chromosome; these dsbs are mended over a period of hours with 100% survival and virtually no mutagenesis. This contrasts with nearly all other organisms in which just a few ionizing radiation induced-dsbs per chromosome are lethal. In this article we present an hypothesis that resistance of D. radiodu rans to ionizing radiation and its ability to mend radiation-induced d sbs are due to a special form of redundancy wherein chromosomes exist in pairs, linked to each other by thousands of four-stranded (Holliday ) junctions. Thus, a dsb is not a lethal event because the identical u ndamaged duplex is nearby, providing an accurate repair template. As a ddressed in this article, much of what is known about D. radiodurans s uggests that it is particularly suited for this proposed novel form of DNA repair.