Practice question

Deinococcus radiodurans, Gram-positive tetrad-forming non-sporulating bacterium isolated from gamma-irradiated canned meat, holds world record for ionizing radiation resistance surviving acute doses exceeding 15 kiloGray, ultraviolet, desiccation and oxidative stress due to multilayered defense systems making it uniquely suited for remediation of nuclear weapons production sites like Hanford and Savannah River where organic solvents such as toluene, trichloroethene and heavy metals co-contaminate radionuclide plumes that kill conventional bioremediation bacteria. Resistance mechanisms include exceptionally efficient homologous recombination mediated by RecA, DdrA single-strand annealing protein, DdrB, extended synthesis-dependent strand annealing reassembling shattered genome from 20 to 30 genome copies per cell in tetrad arrangement preventing misrepair, potent antioxidant protection via manganese-peptide complexes accumulated to 2 millimolar scavenging reactive oxygen species protecting proteins, and condensed toroidal nucleoid limiting diffusion of fragments. Wild type lacks toluene degradation, therefore engineered strains constructed via insertion of merA mercuric reductase reducing Hg2+ to volatile Hg0, todC1C2BA toluene dioxygenase, xylE catechol dioxygenase under radiation-inducible promoter recA and PprI regulator enabling simultaneous detoxification of organic and metal contaminants even during chronic irradiation 60 Gray per hour. Standard laboratory organisms Bacillus subtilis, Pseudomonas aeruginosa and Escherichia coli succumb within 0.5 kiloGray, underscoring Deinococcus advantage.