Switching catalytic activity in the XerCD site-specific recombination machine

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.