Orientation specificity of the Lactococcus lactis Chi site

• Published in: Genes to cells : devoted to molecular & cellular mechanisms Vol. 5; no. 6; pp. 453 - 461
• Main Authors: El Karoui, Meriem, Schaeffer, Matthieu, Biaudet, Véronique, Bolotin, Alexandre, Sorokin, Alexeï, Gruss, Alexandra
• Format: Journal Article
• Language: English
• Published: Oxford BSL Blackwell Science Ltd 01-06-2000; Wiley
• Subjects: Chromosome Inversion; Chromosomes, Bacterial - genetics; DNA Replication; Exodeoxyribonuclease V; Exodeoxyribonucleases - genetics; Exodeoxyribonucleases - metabolism; Gene Frequency; Lactococcus lactis; Lactococcus lactis - genetics; Lactococcus lactis - metabolism; Life Sciences; RecBCD enzyme; Recombination, Genetic; Regulatory Sequences, Nucleic Acid - genetics; LACTOCOCCUS LACTIS
• ISSN: 1356-9597; 1365-2443
• DOI: 10.1046/j.1365-2443.2000.00342.x

Background
In Escherichia coli, the Chi sequence modulates the activity of RecBCD, a powerful double‐stranded (ds) DNA exonuclease/helicase. Chi attenuates RecBCD exonuclease activity and stimulates homologous recombination in an orientation‐dependent manner. ChiEc is frequent and over‐represented 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 ChiLl) and an exonuclease/helicase in the Gram‐positive bacterium Lactococcus lactis. ChiLl and RexAB are functional analogues of ChiEc and RecBCD.
Results
We report that ChiLl attenuates RexAB exonuclease activity and stimulates homologous recombination in an orientation‐dependent manner. Analysis of ChiLl distribution on the L. lactis chromosome reveals that ChiLl is frequent, highly over‐represented, and oriented with respect to the direction of replication.
Conclusion
Our results show that a single orientation of ChiLl interacts with RexAB. The active orientation is preferentially found on the replication leading strand of the L. lactis genome, consistent with a primary role of ChiLl in repair of dsDNA breaks at the replication fork. We propose that orientation‐dependence of Chi activity and over‐representation of Chi sequences on bacterial genomes may be conserved properties of exonuclease/helicase–Chi couples. Other properties of the Chi sequence distribution on the genomes might reflect more specific characteristics of each couple and of the host.