The Croonian Lecture, 1994. Populations, infectious disease and immunity: a very nonlinear world

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Abstract
The Xer site-specific recombination system functions in Escherichia coli to ensure that circular plasmids and chromosomes are in the monomeric state prior to segregation at cell division. Two recombinases, XerC and XerD, bind cooperatively to a recombination site present in the E. coli chromosome and to sites present in natural multicopy plasmids. In addition, recombination at the natural plasmid site cer, present in ColE1, requires the function of two additional accessory proteins, ArgR and PepA. These accessory proteins, along with accessory DNA sequences present in the recombination sites of plasmids are used to ensure that recombination is exclusively intramolecular, converting circular multimers to monomers. Wild-type and mutant recombination proteins have been used to analyse the formation of recombinational synapses and the catalysis of strand exchange in vitro. These experiments demonstrate how the same two recombination proteins can act with different outcomes, depending on the organization of DNA sites at which they act. Moreover, insight into the separate roles of the two recombinases is emerging.