SummaryEscherichia coli DNA polymerase III (Pol III) is one of the best studied replicative DNA polymerases. Here we report the properties of an E. coli mutant that lacks one of the subunits of the Pol III clamp loader complex, Psi ( y y y y ), as a result of the complete inactivation of the holD gene. We show that, in this mutant, chronic induction of the SOS response in a RecFORdependent way leads to lethality at high temperature. The SOS-induced proteins that are lethal in the holD mutant are the specialized DNA polymerases Pol II and Pol IV, combined with the division inhibitor SfiA. Prevention of SOS induction or inactivation of Pol II, Pol IV and SfiA encoding genes allows growth of the holD mutant, although at a reduced rate compared to a wild-type cell. In contrast, the SOS-induced Pol V DNA polymerase does not participate to the lethality of the holD mutant. We conclude that: (i) Psi is essential for efficient replication of the E. coli chromosome; (ii) SOS-induction of specialized DNA polymerases can be lethal in cells in which the replicative polymerase is defective, and (iii) specialized DNA polymerases differ in respect to their access to inactivated replication forks.

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Exponentially growing recA mutant cells of Escherichia coli display pronounced DNA degradation that starts at the sites of DNA damage and depends on RecBCD nuclease (ExoV) activity. As a consequence of this "reckless" DNA degradation, populations of recA mutants contain a large proportion of anucleate cells. We have found that both DNA degradation and anucleate-cell production are efficiently suppressed by mutations in the xonA (sbcB) and sbcD genes. The suppressive effects of these mutations were observed in normally grown, as well as in UV-irradiated, recA cells. The products of the xonA and sbcD genes are known to code for the ExoI and SbcCD nucleases, respectively. Since both xonA and sbcD mutations are required for strong suppression of DNA degradation while individual mutations have only a weak suppressive effect, we infer that ExoI and SbcCD play partially redundant roles in regulating DNA degradation in recA cells. We suggest that their roles might be in processing (blunting) DNA ends, thereby producing suitable substrates for RecBCD binding.

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