The tyrosine family site-specific recombinases, XerCD, function in the conv
ersion of circular dimer replicons to monomers. In the recombining complex
that contains two synapsed recombination sites and two molecules each of Xe
rC and XerD, the DNA strand-exchange reactions are separated in time and sp
ace. XerC initiates recombination to form a Holliday junction intermediate,
which undergoes a conformational change to provide a substrate for strand
exchange by XerD. XerCD are two-domain proteins, whose C-terminal domains c
ontain all of the catalytic residues. We show that XerC or XerD variants la
cking their N-terminal domains are active in recombination when combined wi
th their wild-type partner. Nevertheless, the normal pattern of catalysis i
s dramatically altered; strand exchange by the recombinase variant is stimu
lated, while that by the wild-type partner recombinase is impaired. The pri
mary determinants for the mutant phenotype reside in the region of alpha -h
elix B of XerD. We propose that altered interactions within the recombining
heterotetramer lead to changes in the relative concentrations of the two a
lternative Holliday junction substrates that are recombined by XerC or XerD
, respectively. (C) 2001 Academic Press.