A ring-like nucleoid is not necessary for radioresistance in the Deinococcaceae.
BACKGROUND
Transmission electron microscopy images of Deinococcus radiodurans R1 suggest that the nucleoid of this species exists as a "ring-like" body, and have led to speculation that this structure contributes to the radioresistance of the species. Since extreme radioresistance is characteristic of six other species of Deinococcus, we have attempted to correlate nucleoid morphology and radioresistance by determining whether the genomic DNA of each of these species exhibit similar structures.


RESULTS
The nucleoid morphologies of seven recognized species of Deinococcus, the radioresistant bacterium Rubrobacter radiotolerans, and the more radiosensitive deinococcal relative Thermus aquaticus were evaluated using epifluorescence and deconvolution techniques. Although the nucleoids of Deinococcus murrayi, Deinococcus proteolyticus, Deinococcus radiophilus, and Deinococcus grandis have structures similar to D. radiodurans, the majority of nucleoids found in Deinococcus radiopugnans and Deinococcus geothermalis lack any specific organization. The nucleoid of R. radiotolerans consists of multiple highly condensed spheres of DNA scattered throughout the cell. The genomic DNA of Thermus aquaticus is uniformly distributed throughout the cell.


CONCLUSIONS
There is no obvious relationship between the shape of a species' nucleoid and extreme radioresistance. However, the genomes of all extremely radioresistance species examined are highly condensed relative to more radiosensitive species. Whether DNA in this tightly packed configuration contributes to the radioresistance of these bacteria remains unknown, but this common structural feature appears to limit diffusion of fragments generated post-irradiation even in cells incapable of repairing strand breaks.
Zimmerman
Julie M
JM
Department of Biological Sciences, Louisiana State University and A&M College, Baton Rouge, Louisiana 70803, USA. jzimme5@lsu.edu
Battista
John R
JR
D004269:DNA, Bacterial / Q000528:radiation effects* / Q000648:ultrastructure*; D034301:Deinococcus / Q000528:radiation effects* / Q000648:ultrastructure*; D004307:Dose-Response Relationship, Radiation; D011836:Radiation Tolerance; D013824:Thermus / Q000528:radiation effects / Q000648:ultrastructure
D004269:DNA, Bacterial