An exonuclease I-sensitive DNA repair pathway in Deinococcus radiodurans: a major determinant of radiation resistance


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Deinococcus radiodurans R1 recovering from acute dose of γ radiation shows a biphasic mechanism of DNA double-strand break repair. The possible involvement of microsequence homology-dependent, or non-homologous end joining type mechanisms during initial period followed by RecA-dependent homologous recombination pathways has been suggested for the reconstruction of complete genomes in this microbe. We have exploited the known roles of exonuclease I in DNA recombination to elucidate the nature of recombination involved in DNA double-strand break repair during post-irradiation recovery of D. radiodurans. Transgenic Deinococcus cells expressing exonuclease I functions of Escherichia coli showed significant reduction in γ radiation radioresistance, while the resistance to far-UV and hydrogen peroxide remained unaffected. The overexpression of E. coli exonuclease I in Deinococcus inhibited DNA double-strand break repair. Such cells exhibited normal post-irradiation expression kinetics of RecA, PprA and single-stranded DNA-binding proteins but lacked the divalent cation manganese [(Mn(II)]-dependent protection from γ radiation. The results strongly suggest that 3′ (ρ) 5′ single-stranded DNA ends constitute an important component in recombination pathway involved in DNA double-strand break repair and that absence of sbcB from deinococcal genome may significantly aid its extreme radioresistance phenotype.