Topology of Xer site-specific recombination
Seminar Room 1, Newton Institute
Xer site-specific recombination at cer
converts bacterial plasmid multimers to monomers so that they can be efficiently segregated to both daughter cells at cell division. Recombination is catalysed by the XerCD recombinase acting at 30 bp core sites, and is regulated by the action of accessory proteins on accessory DNA sequences adjacent to the core sites. Recombination normally occurs between sites in direct repeat in a negatively supercoiled circular DNA molecule, and yields two circular products linked together in a right-handed 4-noded catenane with anti-parallel sites. Intramolecular recombination, and recombination between sites in inverted repeat on an unknotted circular substrate do not normally take place. However, recombination between sites in inverted repeat on right-handed torus knots with 5 or more nodes, or between anti-parallel sites on the two rings of a torus catenane with 6 or more nodes is efficient. In each case, recombination adds one additional node to the catenated or knotted substrate. These results are consistent with a model in which the accessory DNA sequences are interwrapped around the accessory proteins so that 3 interdomainal nodes are trapped by synapsis of the core sites by the XerCD recombinase. Recombination between directly repeated sites on an unknotted circular substrate formed the 4-noded catenane product with a linkage change ( ΔLk) of +4. This result is consistent with current models for strand exchange by the XerCD and other tyrosine recombinases, which predict recombination via a Holliday junction intermediate with no net change in total linkage ( ΔLg = Δcat + ΔLk = 0).