engleski

Exonuclease VII is involved in "reckless" DNA degradation in UV-irradiated Escherichia coli

The recA mutants of Escherichia coli exhibit an abnormal DNA degradation that starts at sites of double-strand DNA breaks, and is carried out by RecBCD exonuclease (ExoV). This “reckless” DNA degradation occurs spontaneously in exponentially growing recA cells, and is stimulated by DNA-damaging agents. We have previously found that xonA and sbcD mutations that inactivate exonuclease I (ExoI) and SbcCD nuclease, respectively, markedly suppress “reckless” DNA degradation in UV-irradiated recA cells. In this work, we show that inactivation of exonuclease VII (ExoVII), by a xseA mutation, contributes to attenuation of DNA degradation in UV-irradiated recA mutants. The xseA mutation itself has only a weak effect, however, it acts synergistically with the xonA or sbcD mutations in suppressing “reckless” DNA degradation. The quadruple xseA xonA sbcD recA mutants show no sign of DNA degradation during post-irradiation incubation, suggesting that ExoVII, together with ExoI and SbcCD, play a crucial role in regulating RecBCD-catalyzed chromosome degradation. We propose that these nucleases act on DSBs to prepare blunt DNA ends, the preferred substrates for RecBCD enzyme. In addition, our results show that in UV-irradiated recF recA+ cells, the xseA, xonA, and sbcD mutations do not affect RecBCD-mediated DNA repair, suggesting that ExoVII, ExoI and SbcCD nucleases are not essential for the initial targeting of RecBCD on DSBs. It is possible that the DNA blunting activity provided by ExoVII, ExoI and SbcCD is required for an exchange of RecBCD molecules on dsDNA ends during ongoing “reckless” DNA degradation.

Escherichia coli ; “reckless” DNA degradation ; ExoI ; ExoVII ; SbcCD ; UV irradiation

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