Determinants of selectivity in Xer site-specific recombination

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Xer site-specific recombination involves XerC and XerD recombinases interacting with specific DNA core sites.
• Recombination can be intramolecular (e.g., plasmid cer sites) or both intra- and intermolecular (e.g., E. coli dif site).
• Accessory sequences and proteins are required for intramolecular recombination at plasmid sites.

Purpose of the Study:

• To investigate how altered spacing between XerC- and XerD-binding sites affects recombination selectivity.
• To elucidate the role of accessory factors in ensuring intramolecular recombination.

Main Methods:

• Utilized a model system to manipulate the spacing between XerC- and XerD-binding sites.
• Analyzed changes in recombinase affinity, DNA-protein complex geometry, and cleavage efficiency.
• Correlated these changes with recombination outcomes.

Main Results:

• Increasing the spacing between binding sites from 6 to 8 bp reduced recombinase affinity and altered complex geometry.
• These geometric changes correlated with reduced XerC-mediated cleavage efficiency.
• Selectivity for intramolecular recombination emerged with increased spacing, necessitating accessory factors.

Conclusions:

• Accessory sequences and proteins compensate for reduced recombinase affinity and altered DNA geometry.
• They facilitate the formation of a fixed-geometry nucleoprotein structure essential for intramolecular recombination.
• This mechanism ensures efficient Xer recombination on circular DNA molecules with directly repeated sites.