We reported previously the clonal isolation in normal rat kidney (NRK) cells of two variants of mouse sarcoma virus (MSV) from the Moloney sarcoma-leukaemia complex (MSV-MoLV) (Somers & Kit, 1971). One isolate (MSV-1) has a restricted host range, is capable of transforming NRK cells, but cannot replicate infectious progeny virus. The other isolate (MSV-6) is incapable of either transformation or replication in any cell tested. Recently, it was reported that non-infectious Rous sarcoma virus [RSVα(0)] was deficient in RNA-dependent DNA polymerase activity, but conflicting evidence was reported for DNA-dependent DNA polymerase (Hanafusa & Hanafusa, 1971; Robinson & Robinson, 1971). We examined both DNA polymerase and endonuclease activities in purified particles of MSV-MoLV, MoLV, and in the two clonal isolates, MSV-1 and MSV-6. All preparations contained similar levels of endonuclease activity. However, the DNA polymerase activities of MSV-1 and MSV-6 were 25% and 3%, respectively, of the activity found in MSV-MoLV or MoLV.


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