XER RECOMBINATION IN ESCHERICHIA-COLI - SITE-SPECIFIC DNA TOPOISOMERASE ACTIVITY OF THE XERC AND XERD RECOMBINASES

Xer site-specific recombination functions in maintaining circular repl icons in the monomeric state in Escherichia coli. Two recombinases of the bacteriophage A integrase family, XerC and XerD, are required for recombination at the chromosomal site, dif, and at a range of plasmid- borne sites. Xer recombination core sites contain the 11-base pair bin ding sites for each recombinase separated by a 6 to 8-base pair centra l region. We report that both XerC and XerD act as site-specific type I topoisomerases by relaxing supercoiled plasmids containing a dif sit e. Relaxation by either XerC or XerD occurs in the absence of the part ner recombinase and requires only a single recombination core site, Xe rC or XerD relaxation activities are completely inhibited by the addit ion of the partner recombinase, providing that the DNA recognition seq uence for the inhibiting partner is present.