Topoisomerase II from Chlorella virus PBCV-1 - Characterization of the smallest known type II topoisomerase

Type II topoisomerases, a family of enzymes that govern topological DNA int erconversions, are essential to many cellular processes in eukaryotic organ isms. Because no data are available about the functions of these enzymes in the replication of viruses that infect eukaryotic hosts, this led us to ex press and characterize the first topoisomerase II encoded by one of such vi ruses. Paramecium bursaria chlorella virus 1 (PBCV-1) infects certain chlor ella-like green algae and encodes a 120-kDa protein with a similarity to ty pe II topoisomerases. This protein was expressed in Saccharomyces cerevisia e and was highly active in relaxation of both negatively and positively sup ercoiled plasmid DNA, catenation of plasmid DNA, and decatenation of kineto plast DNA networks. Its optimal activity was determined, and the omission o f Mg2+ or its replacement with other divalent cations abolished DNA relaxat ion, All activities of the recombinant enzyme were ATP dependent. Increasin g salt concentrations shifted DNA relaxation from a normally processive mec hanism to a distributive mode. Thus, even though the PBCV-1 enzyme is consi derably smaller than other eukaryotic topoisomerase II enzymes (whose molec ular masses are typically 160-180 kDa), it displays all the catalytic prope rties expected for a type II topoisomerase.