Exonucleases I, III, and V are required for stability of ColE1-related plasmids in Escherichia coli.

The stability of two ColE1-related plasmids (pRSF2124 and pMB9) was examined in strains of Escherichia coli multiply deficient in exonucleases I (sbcB), III (xthA), or V (recB recC). Any combination of exonuclease I, III, and V deficiency resulted in dramatically decreased stability of both pRSF2124 and pMB9. Inactivation of the RecF pathway by introducing either recF or recJ mutations to the recB recC subcB background resulted in nearly wild-type levels of stability for both plasmids. In contrast, the introduction of uvrD3 uvr-257, uvrE100, or recL152 into the recB21 recC22 sbcB15 strain did not affect plasmid stability. Furthermore, the amount of plasmid DNA recovered from pRSF2124 or pMB9 transformants of a xthA1 sbcB15 strain was strikingly reduced relative to that of a wild-type control. Taken together, these results suggest that some aspect of DNA repair is required for stable maintenance of ColE1-related plasmids in E. coli.