IDENTIFICATION OF THE LACTOCOCCAL EXONUCLEASE RECOMBINASE AND ITS MODULATION BY THE PUTATIVE CHI-SEQUENCE/

Studies of RecBCD-Chi interactions in Escherichia coli have served as a model to understand recombination events in bacteria, However, the e xistence of similar interactions has not been demonstrated in bacteria unrelated to E. coli, We developed an in vivo model to examine compon ents of dsDNA break repair in various microorganisms. Here, we identif y the major exonuclease in Lactococcus lactis, a Gram-positive organis m evolutionarily distant from E. coli, and provide evidence for exonuc lease-Chi interactions. Insertional mutants of L. lactis, screened as exonuclease-deficient, affected a single locus and resulted in UV sens itivity and recombination deficiency, The cloned lactococcal genes (ca lled rexAB) restored UV resistance, recombination proficiency, and the capacity to degrade linear DNA, to an E. coli recBCD mutant, In this contest, DNA degradation is specifically blocked by the putative lacto coccal Chi site (5'-GCGCGTG-3'), but not by the E. coli Chi (5'-GCTGGT GG-3') site. RexAB-mediated recombination was shown to be stimulated a pproximate to 27-fold by lactococcal Chi, Our results reveal that RexA B fulfills the biological roles of RecBCD and indicate that its activi ty is modulated by a short DNA sequence, We speculate that exonuclease /recombinase enzymes whose activities are modulated by short DNA seque nces are widespread among bacteria.