Additive effects of SbcCD and PolX deficiencies in the in vivo repair of DNA double-strand breaks in Deinococcus radiodurans

Orthologs of proteins SbcD (Mre11) and SbcC (Rad50) exist in all kingdoms of life and are involved in a wide variety of DNA repair and maintenance functions, including homologous recombination and nonhomologous end joining. Here, we have inactivated the sbcC and/or sbcD genes of Deinococcus radiodurans , a highly radioresistant bacterium able to mend hundreds of radiation-induced DNA double-strand breaks (DSB). Mutants devoid of the SbcC and/or SbcD proteins displayed reduced survival and presented a delay in kinetics of DSB repair and cell division following γ-irradiation. It has been recently reported that D. radiodurans DNA polymerase X (PolX) possesses a structure-modulated 3′-to-5′ exonuclease activity reminiscent of specific nuclease activities displayed by the SbcCD complex from Escherichia coli . We constructed a double mutant devoid of SbcCD and PolX proteins. The double-mutant Δ sbcCD Δ polX Dr (where Dr indicates D. radiodurans ) bacteria are much more sensitive to γ-irradiation than the single mutants, suggesting that the deinococcal SbcCD and PolX proteins may play important complementary roles in processing damaged DNA ends. We propose that they are part of a backup repair system acting to rescue cells containing DNA lesions that are excessively numerous or difficult to repair.