Orientation specificity of the Lactococcus lactis Chi site

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• The Chi sequence in Escherichia coli influences RecBCD enzyme activity, affecting double-stranded DNA break repair and homologous recombination.
• ChiEc is abundant and over-represented in E. coli's genome, suggesting a significant role in DNA repair.
• A functional analogue, ChiLl, and its associated exonuclease/helicase, RexAB, were identified in Lactococcus lactis.

Purpose of the Study:

• To investigate the functional relationship between ChiLl and RexAB in Lactococcus lactis.
• To determine if ChiLl exhibits orientation-dependent modulation of RexAB activity and homologous recombination.
• To analyze the distribution and orientation of ChiLl on the Lactococcus lactis genome.

Main Methods:

• Enzyme activity assays to measure RexAB exonuclease activity.
• Homologous recombination assays to assess recombination stimulation.
• Bioinformatic analysis of ChiLl distribution on the Lactococcus lactis chromosome.

Main Results:

• ChiLl was found to attenuate RexAB exonuclease activity.
• ChiLl demonstrated orientation-dependent stimulation of homologous recombination.
• ChiLl is frequent, over-represented, and oriented with the direction of replication on the L. lactis chromosome.

Conclusions:

• A specific orientation of ChiLl interacts with RexAB, preferentially located on the replication leading strand.
• This orientation suggests a key role for ChiLl in repairing double-stranded DNA breaks at the replication fork.
• The orientation-dependence and over-representation of Chi sequences are likely conserved features of bacterial exonuclease/helicase-Chi systems.