Nucleic Acids Research, 1992, Vol. 20, No. 23 6159-6166
© 1992
MOLECULAR BIOLOGY |
Recombination of nicked DNA knots by 
resolvase suggests a variant model for the mechanism of strand exchange
Department of Biology, University of Konstanz PO Box 5560, D-7750 Konstanz, Germany
Received October 29, 1992. Accepted November 10, 1992.
Fast and efficient recombination catalyzed by 
resolvase in vitro requires negative DNA supercoiling of plasmid substrates. The current model for recombination suggests that supercoiling is required to drive DNA strand exchange within a synaptic complex by ‘simple rotation’ of DNA-linked resolvase protomers. Surprisingly, DNA knots are recombined efficiently in the absence of supercoiling, whereby the rate of recombination increases with the number of irreducible DNA segment crossings, or nodes, within each substrate knot. Recombination products contain three knot nodes less than substrates, suggesting that a reduction in writhe drives the reaction. However, the proposed protomer rotation model predicts that writhe is not altered during the process of strand transfer but, instead, is reduced only when a synaptic complex disassembles after strand exchange. I present evidence that recombination of knotted and of linear substrates coincides with a disassembly of synaptic complexes. The results lead to a variant model for strand exchange on non-supercoiled substrates in which a specific disassembly of the synaptic complex, triggered by a reduction in writhe, guides the cleaved DNA into the recombinant configuration.