SITE-SPECIFIC RECOMBINATION AND CIRCULAR CHROMOSOME SEGREGATION

The Xer site-specific recombination system functions in Escherichia co li to ensure that circular plasmids and chromosomes are in the monomer ic 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, Ar gR and PepA. These accessory proteins, along with accessory DNA sequen ces 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 hav e been used to analyse the formation of recombinational synapses and t he catalysis of strand exchange in vitro. These experiments demonstrat e how the same two recombination proteins can act with different outco mes, depending on the organization of DNA sites at which they act. Mor eover, insight into the separate roles of the two recombinases is emer ging.