Orientation specificity of the Lactococcus lactis Chi site

Background: In Escherichia coli, the Chi sequence modulates the activity of RecBCD, a powerful double-stranded (ds) DNA exonuclease/helicase. Chi atte nuates RecBCD exonuclease activity and stimulates homologous recombination in an orientation-dependent manner. Chi(Ec) is frequent and over-represente d on its genome, which is thought to be related to its role in dsDNA break repair. We previously identified a Chi-like sequence (referred to as Chi(Ll )) and an exonuclease/helicase in the Gram-positive bacterium Lactococcus l actis. Chi(Ll) and RexAB are functional analogues of Chi(Ec) and RecBCD. Results: We report that Chi(Ll) attenuates RexAB exonuclease activity and s timulates homologous recombination in an orientation-dependent manner. Anal ysis of Chi(Ll) distribution on the L. lactis chromosome reveals that Chi(L l) is frequent, highly over-represented, and oriented with respect to the d irection of replication. Conclusions: Our results show that a single orientation of Chi(Ll) interact s with RexAB. The active orientation is preferentially found on the replica tion leading strand of the L. lactis genome, consistent with a primary role of Chi(Ll) in repair of dsDNA breaks at the replication fork. We propose t hat orientation-dependence of Chi activity and over-representation of Chi s equences on bacterial genomes may be conserved properties of exonuclease/he licase-Chi couples. Other properties of the Chi sequence distribution on th e genomes might reflect more specific characteristics of each couple and of the host.