Extreme radioresistance of Deinococcus radiodurans

Deinococcus radiodurans finds mention in the Guinness Book of World Records as World's Most Radioresistant Bacterium. Discovered in irradiated meat cans in 1952, this microbe can easily survive ionizing radiation doses of 6 kGy and more. It is also resistant to other DNA damaging agents, such as ultraviolet light or mitomycin C and can successfully endure >6 weeks of desiccation or prolonged nutrient deficiency. Deinococcal genome is organised as a toroid ring structure along the cell periphery and has been completely sequenced. The genome harbours around 3200 genes distributed on a large and a small chromosome, one megaplasmid and a small plasmid, i.e. four pieces of circular DNA in every cell. Strangely, Deinococcus does not contain even the normal complement of DNA repair genes found in all other bacteria. Nearly one-third of its genome is unique to the organism and may hold the key to its radioresistance. A great deal of multiplicity and redundancy of DNA is observed in this superbug and greatly aids a highly proficient recombinational DNA repair and consequently high radioresistance of this microbe. Deinococcal DNA repair is further strengthened by presence of constitutively high levels of enzymatic and non-enzymatic ROS scavengers and supplemented by a robust mechanism of protein recycling to repair and restore the radiation damaged proteome. The organism provides unique opportunities for basic and applied research, especially related to DNA repair and aimed at engineering the microbe for bioremediation of metals and organic solvents from nuclear waste. The work at BARC has led to (a) identification and characterization of several unique DNA repair genes from this organism and their function as a multiprotein complex, (b) discovery of pyrroloquinoline quinone (PQQ) as a novel antioxidant and signaling molecule involved in DNA repair, (c) construction of proteomic maps and revelation of the proteome kinetics and dynamics during post-irradiation recovery of this bacterium. We have also genetically engineered this microbe successfully to bioremediate and recover uranium from aqueous acidic and alkaline solutions, in high radiation environments. Highlights of these studies will be presented. (author)