The Radioresistance of Deinococcus Radiodurans.
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- Biology; microbiology; Biology; molecular
thesis / dissertation description
Deinococcus radiodurans is a remarkably ionizing radiation resistant bacterium. As a vegetative cell it is capable of withstanding 5,000 Gy ionizing radiation without evidence of death or mutation. D. radiodurans survives irradiation by repairing DNA damage via an extremely efficient DNA repair system. However, this DNA repair system has not been completely defined. In addition, the selective process allowing the evolution of high level radioresistance has remained a question. This dissertation bolsters the understanding of the DNA repair system of D. radiodurans by characterizing a collection of ionizing radiation sensitive (IRS) mutants. The level of radioresistance is quantified and transformation ability is assessed for each IRS mutant. A new subset of mutants slow to recover from irradiation is identified. The IRS mutant collection is organized into sixteen linkage groups. Using linkage group data, the order of mutations within the pol linkage group is determined and the distance between cotransformed markers is found to be unusually small. The irrI mutant within linkage group I is observed using pulsed-field gel electrophoresis (PFGE) to examine mutant DNA during a post-irradiation recovery time course. This information in conjunction with survival data provides evidence that the irrI gene product inhibits DNA degradation at DNA gaps or double-strand breaks (dsbs). This dissertation also provides evidence which indicates a selection process by which extreme radioresistance mechanisms could have evolved. A correlation is found between desiccation sensitivity and ionizing radiation sensitivity within the IRS mutant collection. PFGE observation of desiccated and ionizing radiation irradiated cell samples demonstrate that the DNA within these cell samples contains multiple dsbs. Because desiccation and ionizing radiation cause a similar pattern of DNA damage in the form of dsbs, the DNA repair mechanism used to survive ionizing radiation exposure could have evolved in response to periods of desiccation.