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 gamma-irradiation. It has been recently reported that D. radiodurans DNA polymerase X (PoIX) 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 PoIX proteins. The double-mutant Delta sbcCD Delta poly(Dr) (where Dr indicates D. radiodurans) bacteria are much more sensitive to gamma-irradiation than the single mutants, suggesting that the deinococcal SbcCD and PoIX 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.