Presence of Type C Particles Containing Reverse Transcriptase in L1210 Leukemia2

Abstract

A murine L1210 leukemia cell line was grown either in vitro or in vivo by ip injection of 10⁶ cells/mouse in C57BL × DBA)F₁ mice. It contained particles that resembled murine leukemia virus. Electron microscopic examination of the cells showed both intracytoplasmic and budding type C particles. Similar particles were also seen in the high-speed pellet (90,000 × g) obtained from both mouse ascitic fluid and the supernatant from cells grown in tissue culture. When the high-speed pellet was centrifuged in a sucrose density gradient, particles banding at densities 1.16–1.18 g/ml were detected. The virus appeared to be N-tropic. These type C particles contained an RNA-dependent DNA polymerase (reverse transcriptase) activity and performed endogenous DNA synthesis. The enzyme was purified from the cells and virus particles; these enzymes were indistinguishable in their properties. The molecular weight, as determined by velocity sedimentation and gel filtration, was approximately 70,000. Primer template preference studies indicated that the purified enzyme utilized synthetic ribo templates better than synthetic deoxyribo templates. It could also utilize (dG)≈₁₅ (Cm)_n, a primer template specific for reverse transcriptase. The enzyme could transcribe heteropolymeric regions of 70S RNA from Rauscher murine leukemia virus (R-MuLV) and had RNaseH activity as well. The enzyme activity was inhibited by antibody directed against the reverse transcriptase from R-MuLV. The biophysical, biochemical, and immunologic properties of the purified enzyme resembled those of the reverse transcriptase from R-MuLV.