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Astrocytes in cultures of brain cells from fetal or newborn hamsters undergo neoplastic transformation after infection with simian virus 40 or polyoma virus. Subcutaneous or intracerebral inoculation of the transformed brain cells into newborn or adult hamsters produces progressively enlarging astrocytomas at the sites of injection. Astrocytomas produced by polyomatransformed cell lines are histologically better differentiated, but grow more rapidly and metastasize more frequently, than astrocytomas produced by cell lines transformed by simian virus 40. These observations make available in vitro models of virus-induced oncogenesis in astrocytes and provide simple techniques for obtaining astrocytoma cell lines suitable for screening studies of chemical agents effective against astrocytomas.

Polyoma virus particles were purified by equilibrium centrifugation in CsCl. Particles from three regions of the density gradient were examined for infectivity, for their ability to induce expanded pools of deoxyribonucleic acid (DNA) precursors, and for their ability to stimulate the synthesis of DNA. The most infectious population of particles, the virions, having a buoyant density of 1.33 g/ml, gave the greatest stimulation of the DNA-synthesizing apparatus of mouse embryo cells. Empty particles at density 1.29 g/ml had no DNA stimulatory activity. A population of particles of intermediate density, referred to as pseudovirions, was also much less active than virions in stimulating DNA synthesis, and the limited stimulatory activity of the latter fraction may be accounted for by its measured contamination with infective particles.

Randomly labeled RNA's from animal cells either productively infected or transformed by polyoma virus were used to measure virus DNA transcription. During lytic infection, 50 percent of polyoma virus DNA was expressed. In two different polyoma-transformed hamster cell lines, the same 20 percent of the virus genome was transcribed. About 10 percent of polyoma DNA was expressed in the one mouse transformed cell line examined.

A complex containing polyoma (py) DNA and protein (py complex) was isolated from polyoma-infected mouse-cell cultures. The complex sedimented unimodally at about 55 S. When labeled for long periods (2-3 hr) between 20 and 40 hr after infection, most of the [(3)H]DNA in the py complex was in the form of covalently closed, circular polyoma DNA (component I). When labeled for 5 min, the [(3)H]DNA in the py complex was nicked in one or both of the strands, as shown by alkaline sucrose gradient centrifugation. Under all conditions studied, no free py DNA was extracted from mouse cells by the two methods described.