Evidence for type‐C retrovirus production by Burkitt's lymphoma‐derived cell line

Burkitt's lymphoma cell line, P3HR‐1, was found to secrete virions with properties of known type‐C RNA tumor viruses. The viral particles had a buoyant density of 1.16 g/ml in sucrose gradients and contained a highmolecular‐weight RNA and an RNA‐instructed DMA polymerase. The viral polymerase was active in an endogenous reaction requiring the presence of the four deoxyriboside triphosphates and manganese ions, and was sensitive to RNase. The DNA product of the endogenous reaction specifically hybridized to P3HR‐I viral 60 to 70S RNA. Electron microscopic examination of ultrathin sections of P3HR‐I cells revealed immature, mature and budding virions typical of type‐C retroviridae. Nucleic acid hybridization assays showed no sequence homology between the genome of P3HR‐I virus and avian myeloblastosis virus, murine oncornaviruses, simian sarcoma virus or RD₁₁₄ virus.