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Swapping DNA strands and sensing homology without branch migration in lambda site-specific recombination
S E Nunes-Düby1, M A Azaro, A Landy
1Department of Biology and Medicine, Brown University, Providence, Rhode Island 02912, USA.
Current Biology : CB
|February 1, 1995
Summary
Bacteriophage lambda site-specific recombination does not primarily rely on extensive branch migration. Instead, sequence homology is sensed during strand annealing, leading to a new mechanistic model.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- Site-specific recombinases often resolve Holliday junction intermediates.
- Previous models proposed branch migration across homology regions for recombination.
Purpose of the Study:
- Investigate the role of branch migration in bacteriophage lambda site-specific recombination.
- Determine the mechanism of Holliday junction resolution by lambda integrase.
Main Methods:
- Utilized synthetic lambda att-site Holliday junctions with sequence heterologies.
- Analyzed resolution bias by lambda integrase (Int) with varying branch point positions.
- Performed isolated strand-joining reactions with and without base complementarity.
Main Results:
- Optimal junction resolution occurred near the middle of the overlap region, not the ends.
- Minor shifts in branch point position significantly altered resolution bias.
- Branch migration appears limited to 1-3 base pairs within the central overlap region.
- Proposed a new model involving symmetrical nucleotide swaps and a central isomerization step.
- Sequence homology is likely sensed during the annealing step before strand joining.
Conclusions:
- Holliday junction resolution in lambda site-specific recombination is not primarily driven by branch migration.
- Lambda Int cleaves Holliday junctions near the crossover, similar to endonucleases in homologous recombination.