DNA repair in the extremely radioresistant bacterium <i>Deinococcus radiodurans</i>

Summary Deinococcus radiodurans and other members of the same genus share extraordinary resistance to the lethal and mutagenic effects of ionizing and u.v. radiation and to many other agents that damage DNA. While it is known that this resistance is due to exceedingly efficient DNA repair, the molecular mechanisms responsible remain poorly understood. Following very high exposures to u.v. irradiation (e.g. 500 Jm−2, which is non‐lethal to D. radiodurans), this organism carries out extremely efficient excision repair accomplished by two separate nucleotide excision repair pathways acting simultaneously. One pathway requires the uvrA gene and appears similar to the UvrABC excinuclease pathway defined in Escherichia coli. The other excision repair pathway is specific for u.v. dimeric photoproducts, but is not mediated by a pyrimidine dimer DNA glycosylase. Instead, it is initiated by a second bona fide endonuclease that may recognize both pyrimidine dimers and pyrimidine‐(6–4)pyrimidones. After high doses of ionizing‐radiation (e.g. 1.5Mrad), D. radiodurans can mend &gt;100 double‐strand breaks (dsb) per chromosome without lethality or mutagenesis. Both dsb mending and survival are re