Successful segregation of circular chromosomes in Escherichia coli requires
that dimeric replicons, produced by homologous recombination, are converte
d to monomers prior to cell division. The Xer site-specific recombination s
ystem uses two related tyrosine recombinases, XerC and XerD, to catalyze re
solution of circular dimers at the chromosomal site, dif, A 33-base pair DN
A fragment containing the 28-base pair minimal dif site is sufficient for t
he recombinases to mediate both inter- and intramolecular site-specific rec
ombination in vivo. We show that Xer-mediated intermolecular recombination
in vitro between nicked linear dif "suicide" substrates and supercoiled pla
smid DNA containing dif is initiated by XerC, Furthermore, on the appropria
te substrate, the nicked Holliday junction intermediate formed by XerC is c
onverted to a linear product by a subsequent single XerD-mediated strand ex
change. We also demonstrate that a XerC homologue from Pseudomonas aerugino
sa stimulates strand cleavage by XerD on a nicked linear substrate and prom
otes initiation of strand exchange by XerD in an intermolecular reaction be
tween linear and supercoiled DNA, thereby reversing the normal order of str
and exchanges.