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 interconversions, are essential to many cellular processes in eukaryotic organisms. Because no data are available about the functions of these enzymes in the replication of viruses that infect eukaryotic hosts, this led us to express and characterize the first topoisomerase II encoded by one of such viruses. Paramecium bursaria chlorella virus 1 (PBCV-1) infects certain chlorella-like green algae and encodes a 120-kDa protein with a similarity to type II topoisomerases. This protein was expressed in Saccharomyces cerevisiae and was highly active in relaxation of both negatively and positively supercoiled plasmid DNA, catenation of plasmid DNA, and decatenation of kinetoplast DNA networks. Its optimal activity was determined, and the omission of Mg²⁺ or its replacement with other divalent cations abolished DNA relaxation. All activities of the recombinant enzyme were ATP dependent. Increasing salt concentrations shifted DNA relaxation from a normally processive mechanism to a distributive mode. Thus, even though the PBCV-1 enzyme is considerably smaller than other eukaryotic topoisomerase II enzymes (whose molecular masses are typically 160-180 kDa), it displays all the catalytic properties expected for a type II topoisomerase.