A ParE-ParC fusion protein is a functional topoisomerase

Type II topoisomerases are responsible for DNA unlinking during DNA replica tion and chromosome segregation. Although eukaryotic enzymes are homodimers and prokaryotic enzymes are heterotetramers, both prokaryotic and eukaryot ic type II topoisomerases belong to a single protein family. The amino- and carboxyl-terminal domains of eukaryotic enzymes are homologous to the ATP- binding and catalytic subunits of prokaryotic enzymes, respectively. Topois omerase TV, a prokaryotic type II topoisomerase, consists of the ATP-bindin g subunit, ParE, and the catalytic subunit, ParC. We have joined the coding regions of parE and parC in frame and constructed a fusion protein of the two subunits of topoisomerase IV. This fusion protein, ParEC, can catalyze both decatenation and relaxation reactions. The ParEC protein is also capab le of decatenating replicating daughter DNA molecules during oriC DNA repli cation in vitro. Furthermore, the fusion gene, parEC, complements the tempe rature-sensitive growth of both par-C and parE strains, indicating that the ParEC protein can substitute for topoisomerase IV in vivo. These results d emonstrate that a fusion protein of the two subunits of topoisomerase TV is a functional topoisomerase. Thus, a heterotetrameric type II topoisomerase can be converted into a homodimeric type II topoisomerase by gene fusion.