Evidence for Both 3' and 5' Single-Strand DNA Ends in Intermediates in Chi-Stimulated Recombination in Vivo

This paper focuses on elucidation of the structures of intermediates in recombination stimulated by Chi recombination hotspots in vivo. We report that null mutations in genes encoding single-strand exonucleases of 3' polarity, Exonuclease I (Exo I), of 5' polarity, RecJ, and of both polarities, Exo VII, alter the ability of Chi sites to promote recombination, and diminish the frequency of recombination. Maximal effects occur when single-strand exonucleases of both polarities are eliminated. These data imply that 3' and 5' single-strand DNA ends, the substrates for these exonucleases, exist in bona fide, product-generating intermediates in Chi-stimulated recombination in vivo. These results also identify three new proteins not known previously to affect RecBCD-mediated recombination.