DNA topoisomerase activity associated with simian virus 40 nucleoprotein complexes

Simian virus 40 (SV40) chromatin extracted from nuclei of infected monkey cells (CV1) was sedimented in neutral sucrose gradients, before and after digestion with bovine pancreatic RNase I-A or DNase I. DNA topoisomerase (TI) activity was found associated with RNase-resistant, DNase-sensitive SV40 nucleoprotein complexes. After polyacrylamide gel electrophoresis, a number of proteins with a molecular mass between 40 and 70 kDa were seen at the level of viral DNA peaks, some of which may represent catalytically active breakdown products of the TI enzyme. Large protein complexes were observed under the electron microscope in association with the viral chromosomes and appear to correspond to the SV40 DNA replication complex, including TI. Our results suggest that TI activity is indeed associated with the viral minichromosomes undergoing replication in vivo.