Biochemical properties of endogenous rat C-type viruses

The cellular restriction to persistence of inducible C-type viruses of at least two rat strains can be overcome in actively in vitro replicating and/or spontaneously transforming rat embryo cells. Both, SD-RaLV and W-RaLV particles are morphologically and biochemically related to known oncorna viruses: They are of C-type, have an identical buoyant density of 1.15 g/cm3 in sucrose gradients and contain high-molecular-weight RNA. The RaLV-associated DNA polymerase exhibit both exogenous (polynucleotide-templated) and endogenous (viral RNA-templated) reactivity. The enzyme strongly preferred Mn++ over Mg++ ions (Mg++/Mn++ ratio of 0.04 to 0.07), with maximum reactivity at 0.1 mM MnCl2 concentration. The molecular hybridization experiments revealed the endogenous nature of the SD-RaLV and the W-RaLV. From the competition hybridization assay the number of proviral sequences could be calculated to be 84 per haploid cell genome. The role of endogenous rat viruses in the "spontaneous" in vitro transformation is discussed.