Control of genetic stability in Escherichia coli:: the SbcB 3′-5′ exonuclease suppresses illegitimate recombination promoted by the RecE 5′-3′ exonuclease

Background: The Escherichia coli sbcB gene, which codes for a 3'-5' exonuclease, ExoI, is known to suppress illegitimate recombination. In contrast, the recE gene, which codes for a 5'-3' exonuclease, Exo VIII promotes joining between DNA ends having short stretches of homology. Therefore, it seems likely that the 3'-5' and 5'-3' exonucleases regulate genetic instability that is mediated by illegitimate recombination. However, there has been little evidence to substantiate the involvement of exonuclease activity in the promotion and suppression of illegitimate recombination. Results: Using a plasmid system for the analysis of deletion formation, we first demonstrated that deletion formation is increased by the sbcA mutation, which activates the expression of RecE 5'-3' exonuclease. It is thought that DNA ends having 3'-single stranded overhangs are important for illegitimate recombination. Next, we found that a large supply of SbcB 3'-5' exonuclease suppresses the deletion formation enhanced by the RecE exonuclease. Moreover, the SbcB exonuclease even suppressed deletion formation in cells not expressing RecE exonuclease. Conclusion: We conclude that DNA ends with 3'-overhangs produced by 5'-3' dsDNA exonuclease activity are proficient for illegitimate recombination, while blunt DNA ends produced by 3'-5' ssDNA exonuclease activity are deficient for illegitimate recombination. Therefore, both exonucleases may play important roles in genetic stability by controlling end-joining between DNA molecules.