Isolation of a polyoma-nucleoprotein complex from infected mouse-cell cultures.

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.