Abstract
BackgroundTransmission 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.ResultsThe 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.ConclusionThere 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.
Highlights
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
The Deinococcaceae are a small family within the domain Bacteria that are distinguished by their ability to tolerate DNA double strand breaks [1,2]
The nucleoid of Deinococcus radiodurans R1 Sequential digital images were collected at 100 nm increments from stationary phase cultures of D. radiodurans R1
Summary
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. Ionizing radiation generates an array of DNA damage in the target cell, including many types of base damage, single, and double strand breaks [6]. Of these types of damage, DNA double strand breaks (DSBs) are considered the greatest threat to cell viability, and an excessive number of DSBs (page number not for citation purposes). D. radiodurans survives doses of ionizing radiation that generate greater than 100 DSBs per genome without mutation or loss of viability [8,9,10]. It is the ability to endure and accurately repair these lesions that set the deinococci apart from other species
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