Cell-free assembly of a polyoma-like particle from empty capsids and DNA.

A polyoma-like particle (PLP) is formed when polyoma DNA and purified empty capsids are incubated in a cell-free system. The DNA of this new particle is protected against the action of pancreatic DNase. The density of the purified PLP in CsCl is 1.32 g/cm3, which is intermediate between that of polyoma virions (1.34 g/cm3) and empty capsids (1.29 g/cm3). Purified PLP sediments at 190 S in sucrose and is stable in solutions of high ionic strength. When the DNA is extracted from PLP by the use of detergent and phenol, it is found to be doublestranded with a molecular weight of approximately 1.1 x 10(6). The particles are stable in CsCl at 4 degrees for at least 5 months. Electron micrographs indicate that highly purified PLPs stained with 2% PTA have the same appearance as polyoma capsids. Neither aggregates nor complexes bound by loose ionic bonds appear reasonable to explain these results. The evidence indicates that the DNA of this new polyoma-like particle, made under cell-free conditions, is protected by the capsid.