DNA TOPOISOMERASE ACTIVITY ASSOCIATED WITH SIMIAN-VIRUS-40 NUCLEOPROTEIN COMPLEXES

Simian virus 40 (SV40) chromatin extracted from nuclei of infected mon key cells (CV1) was sedimented in neutral sucrose gradients, before an d after digestion with bovine pancreatic RNase I-A or DNase I. DNA top oisomerase (TI) activity was found associated with RNase-resistant, DN ase-sensitive SV40 nucleoprotein complexes. After polyacrylamide gel e lectrophoresis, 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 ma y 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 th e SV40 DNA replication complex, including TI. Our results suggest that TI activity is indeed associated with the viral minichromosomes under going replication in vivo.