title: Exonuclease VII is involved in "reckless" DNA degradation in UV-irradiated Escherichia coli.
abstract: The recA mutants of Escherichia coli exhibit an abnormal DNA degradation that starts at sites of double-strand DNA breaks (DSBs), and is mediated 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 the xonA and sbcD mutations, which inactivate exonuclease I (ExoI) and SbcCD nuclease, respectively, markedly suppress "reckless" DNA degradation in UV-irradiated recA cells. In the present work, we show that inactivation of exonuclease VII (ExoVII) by an 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, plays a crucial role in regulating RecBCD-catalyzed chromosome degradation. We propose that these nucleases act on DSBs to create blunt DNA ends, the preferred substrates for the 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 to 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.
authors.affiliation: Division of Molecular Biology, Ruđer Bošković Institute, Bijenička, Zagreb, Croatia.
authors.forename: Jelena
authors.initials: J
authors.lastname: Repar
authors.forename: Nina
authors.initials: N
authors.lastname: Briški
authors.forename: Maja
authors.initials: M
authors.lastname: Buljubašić
authors.forename: Ksenija
authors.initials: K
authors.lastname: Zahradka
authors.forename: Davor
authors.initials: D
authors.lastname: Zahradka
mesh_terms: D053903:DNA Breaks, Double-Stranded; D053938:DNA Fragmentation / Q000528:radiation effects; D004260:DNA Repair; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics* / Q000528:radiation effects; D029968:Escherichia coli Proteins / Q000378:metabolism; D005090:Exodeoxyribonucleases / Q000378:metabolism*; D005092:Exonucleases / Q000378:metabolism; D009154:Mutation; D014466:Ultraviolet Rays
chemical_list: D029968:Escherichia coli Proteins; D005090:Exodeoxyribonucleases; D005092:Exonucleases; C081229:sbcD protein, E coli; C028450:exodeoxyribonuclease I; C026870:exodeoxyribonuclease VII