Further studies on the incorporation of cell DNA into polyoma-related particles

Abstract

Polyoma infection of mouse kidney cells, whose DNA was labeled with radioactive thymidine before the time of infection, resulted in the appearance of radioactive hemagglutinating particles. The radioactive DNA extracted from these particles hybridized predominantly with uninfected mouse cell DNA and displayed in CsCl solution a buoyant density (1.702 g/ml) which is characteristic of mouse DNA. The major part of the radioactive cell DNA sedimented together with the “slow” component of polyoma DNA, with an estimated sedimentation coefficient of 14.6 S. After equilibrium centrifugation in a CsCl density gradient, the radioactively labeled particles (from cells labeled prior to infection) were broadly distributed on the lighter side of the main band of plaque-forming particles, with a peak which was approximately 0.01 density units lighter. A band centrifugation method is described which moderately enhanced the separation of the lighter particles. Band centrifugation of the particles produced in cells labeled with radioactive thymidine from hour 22 to hour 70 post-infection, revealed the presence of lighter particles whose radioactive DNA hybridized with uninfected mouse cell DNA. It is concluded (1) that both the cell DNA which exists at the time of infection, as well as that made after infection, is incorporated into a minor fraction of polyoma-related, hemagglutinating particles; (2) that most of the encapsidated cell DNA, on the basis of the results of DNA-DNA hybridization tests, does not represent a highly specific segment of the cellular genome; (3) that some of the aberrant particles contain a 3 × 10 6 molecular weight fragment of cell DNA and no 20 S polyoma DNA.