Identification and characterization of fast-sedimenting SV40 nucleoprotein complexes.

Simian virus 40 (SV40) nucleoprotein complexes were extracted from nuclei of infected monkey cells and fractionated on neutral sucrose density gradients. Four forms of SV40 chromatin were distinguished by their labeling kinetics and apparent sedimentation coefficients: Replicating viral DNA in 95 S SV40 chromatin was converted to 75 S chromatin when viral DNA replication was completed. A portion of the 75 S chromatin was rapidly assembled into 200 S nucleoprotein complexes and subsequently into 240 S complexes. The 75 S and the fast-sedimenting complexes were further characterized and compared with mature SV40 virus by equilibrium density centrifugation, plaque assay, and electron microscopy. These experiments suggest that the fast-sedimenting forms may represent virion precursors or a subclass of salt-labile virions, but they contain little or no mature virus.