Relating primary structure to function in the Escherichia coli XerD site-specific recombinase.

XerC and XerD are related 298-amino-acid site-specific recombinases, each of which is responsible for the exchange of one pair of strands in Xer recombination. Both recombinases encode functions necessary for sequence-specific DNA-binding, co-operative XerC/D interactions, synapsis and catalysis. These functions were related to the primary amino acid sequence by constructing and analysing internal and C-terminal XerD deletions. An XerD derivative containing residues 1-233 was proficient in specific DNA binding, but did not interact co-operatively with XerC. Deletion of a further five C-terminal amino acids abolished binding to DNA. Proteins deleted for residues 32-88 and for residues 145-159 were deficient in DNA binding. Deletion of residues 244-281, a region containing amino acids necessary for catalysis, gave a protein that bound to DNA. An XerD derivative containing residues 1-268 retained co-operative interactions with XerC; nevertheless, it did not support XerC strand exchange and was defective in XerD catalysis. Residues 1-283 retain a functional catalytic active site, though a protein lacking the five C-terminal amino acids was still unable to mediate normal in vivo recombination, indicating that these residues are needed for a function that is not directly related to DNA binding or catalysis.