Abstract
Transmission electron microscopy revealed that the nucleoid of the extremely radioresistant bacteria Deinococcus radiodurans may adopt an unusual ring shape. This led to the hypothesis that the tight toroidal package of the D. radiodurans genome might contribute to radioresistance by preventing diffusion of ends of double-stranded DNA breaks. The molecular arrangement of DNA in the nucleoid, which must be determined to test this hypothesis, is not discernible by conventional methods of electron microscopy. We have applied cryoelectron microscopy of vitreous sections and found that the DNA arrangement in D. radiodurans differs from toroidal spooling. Diffuse coralline nucleoids of exponentially growing D. radiodurans do not reveal any particular molecular order. Electron-dense granules are generally observed in the centers of nucleoids. In stationary-phase cells, the nucleoid segregates from cytoplasm and DNA filaments show locally parallel arrangements, with increasing aspects of cholesteric liquid crystalline phase upon prolonged starvation. The relevance of the observed nucleoid organization to the radiation resistance of D. radiodurans is discussed.