Topoisomerase II from Chlorella virus PBCV-1 has an exceptionally high DNAcleavage activity

Chlorella virus PBCV-1 topoisomerase II is the only functional type II enzy me known to be encoded by a virus that infects eukaryotic cells. However, i t has not been established whether the protein is expressed following viral infection or whether the enzyme has any catalytic features that distinguis h it from cellular type II topoisomerases. Therefore, the present study cha racterized the physiological expression of PBCV-1 topoisomerase II and indi vidual reaction steps catalyzed by the enzyme. Results indicate that the to poisomerase II gene is widely distributed among Chlorella viruses and that the protein is expressed 60-90 min after viral infection of algal cells. Fu rthermore, the enzyme has an extremely high DNA cleavage activity that sets it apart from all known eukaryotic type II topoisomerases. Levels of DNA s cission generated by the viral enzyme are similar to 30 times greater than those observed with human topoisomerase II alpha, The high levels of cleava ge are not due to inordinately tight enzyme-DNA binding or to impaired DNA religation, Thus, they most likely reflect an elevated forward rate of scis sion. The robust DNA cleavage activity of PBCV-1 topoisomerase II provides a unique tool for studying the catalytic functions of type II topoisomerase s.