Site-specific recombination permits highly precise and accurate programmed genetic rearrangements in both prokaryotes and eucaryotes. The Xer site-specific recombination system corrects the potential damage of homologous recombination between circular DNA molecules; homologous recombination can generate dimeric molecules which cannot be segregated efficiently to daughter cells; Xer recombination converts such dimers to monomers. This system uses two related recombinases, XerC and XerD, which catalyse exchanges at a family of sites presents on several plasmids and on the E. coli chromosome. This system appears to be highly conserved amongst bacteria. The aim of this project is to gain further insight into the mechanism of Xer recombination at the chromosomal dif site, and particulary to understand the specific requirements for recombination at this site compared with sites present in naturally occurring plasmids. This study will involve biochemical, genetic and physiological studies. First, the characterization of in vitro recombination reactions using dif substrates and purified recombination proteins will be undertaken. Then site-directed mutagenesis will be used to identify the determinants for dif activity and to study their functional relevance in chromosmal segregation.