Studies of RecBCD-Chi interactions in @entity526 have served as a model to understand recombination events in bacteria. However, the existence of similar interactions has not been demonstrated in bacteria unrelated to @entity526 . We developed an in vivo model to examine components of dsDNA break repair in various microorganisms. Here, we identify the major exonuclease in @entity7878 , a Gram-positive organism evolutionarily distant from @entity526 , and provide evidence for exonuclease-Chi interactions. Insertional mutants of @entity7878 , screened as exonuclease-deficient, affected a single locus and resulted in UV sensitivity and recombination deficiency. The cloned lactococcal genes (called rexAB) restored UV resistance, recombination proficiency, and the capacity to degrade linear DNA, to an @entity526 recBCD mutant. In this context, DNA degradation is specifically blocked by the putative lactococcal Chi site (5'-GCGCGTG-3'), but not by the @entity526 Chi (5'-GCTGGTGG-3') site. RexAB-mediated recombination was shown to be stimulated approximately 27-fold by lactococcal Chi. Our results reveal that RexAB fulfills the biological roles of RecBCD and indicate that its activity is modulated by a short DNA sequence. We speculate that exonuclease/ @entity43598 enzymes whose activities are modulated by short DNA sequences are widespread among bacteria.

Identification of the lactococcal exonuclease/ XXXX and its modulation by the putative Chi sequence.

@entity7878 :: ('1358', 'Species') :: ['Lactococcus lactis', 'L. lactis'] @entity43598 :: ('20468735', 'Gene') :: ['recombinase'] @entity526 :: ('562', 'Species') :: ['Escherichia coli', 'E. coli']
@entity43598 :: (20468735,Gene) :: ['recombinase']
