Genetic Recombination in Escherichia coli: The Role of Exonuclease I

The indirect suppression of recB^- and recB^-recC^- mutations by the sbcB^- allele is caused by the loss of a nuclease active on denatured DNA. Results from enzyme purifications and studies with a specific antiserum demonstrate that the activity present in sbcB⁺ strains, and lost in sbcB^- strains, is exonuclease I. It is likely that sbcB is the structural gene for exonuclease I. The loss of exonuclease I activity restores the recombination proficiency of Escherichia coli cells that has been lost by mutations in the recB and/or recC genes. This indicates that in the absence of the recB-recC-determined enzyme, exonuclease I prevents recombination. Hypothetical pathways illustrating this conclusion are presented.