Relaxases are so named because the single-stranded DNA nick that they catalyze lead to relaxation of helical tension.
This means that they use a metal ion to aid the transfer of an ester bond from the DNA phosphodiester backbone to a catalytic tyrosine side chain, resulting in a long-lived covalent phosphotyrosine intermediate that essentially unified the nicked DNA strand and the enzyme as one molecule.
These revealed compact molecules composed of five-stranded, antiparallel beta sheet cores and peripheral alpha helices.
A histidine-rich motif, previously identified by sequence conservation, was shown to be a metal ion binding site located on the beta sheet core, nearby the carboxy-terminal catalytic tyrosine residue.
This means that they share a general fold, but the amino-terminal sequence of one is homologous to the C-terminus of the other, and vice versa.