Structure and Functions of the Kirsten Murine Sarcoma Virus Genome: Molecular Cloning of Biologically Active Kirsten Murine Sarcoma Virus DNA

The unintegrated closed circular form of viral DNA prepared from NIH3T3 cells infected with Kirsten murine sarcoma virus was cloned into bacterial plasmid pBR322. The closed circular DNA, which consisted of two different-sized populations, was enriched from the virus-infected cells, linearized with Bam HI, and inserted into pBR322 DNA. Four different recombinant DNAs (clones 2, 4, 6, and 7) were obtained, and a physical map of each was constructed by using various restriction enzymes. Clone 4 DNA had the largest insertion, corresponding to a complete copy of the linear DNA. This suggested that this insertion contained two copies of the 0.55-kilobase pair long terminal redundant sequence. Clone 2 and clone 6 insertion DNAs had deletions of 0.2 and 0.5 kilobase pair, respectively, which mapped near the right end (3′ side of viral RNA) of the linear DNA. Clone 7 DNA appeared to have a deletion of a single copy of the large terminal redundant sequence. Transfection of BALB3T3 cells with the clone 4 DNA insertion showed that this DNA had transforming activity. The efficiency of transfection with clone 4 Kirsten murine sarcoma virus DNA was enhanced eightfold by inserting Eco RI-cleaved viral DNA into the Eco RI site of pBR322. The Eco RI-inserted DNA produced foci with single-hit kinetics, suggesting that a single molecule of Kirsten murine sarcoma virus DNA can induce transformation. Results of transfections with Eco RI-inserted Kirsten murine sarcoma virus DNA cleaved with various restriction enzymes suggested that the first 3.3-kilobase pair region at the left end of the linear DNA is important for the initiation of transformation or maintenance of transformation or both.