Molecular cloning and characterization of a leukemia-inducing myeloproliferative sarcoma virus and two of its temperature-sensitive mutants

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Abstract
The myeloproliferative sarcoma virus (MPSV) induces extensive hematopoietic changes, including spleen foci in adult mice, and transforms fibroblasts in vitro. Rat kidney NRK nonproducer cell lines of MPSV and its temperature-sensitive mutants were analyzed by restriction enzyme digestion and Southern blotting. EcoRI fragments containing the proviral DNA of MPSV and 2 temperature-sensitive mutants and rat cellular sequences homologous to c-mos were molecularly cloned. By comparing restriction enzyme cleavage sites, it was shown that the MPSV genome consists only of sequences related either to Moloney murine leukemia virus or to the c-mos mouse oncogenic sequences. Two regions of fragment heterogeneity were observed: in the defective pol gene, where MPSV and the 2 cloned temperature-sensitive mutants were different from Moloney murine sarcoma virus and from each other, although MPSV wild-type retained more of the pol gene than any of the Moloney murine sarcoma virus isolates; in the area 3'' to the mos gene, which was identical in MPSV and its temperature-sensitive mutants but different from other Moloney murine sarcoma virus variants. Transfection of cloned MPSV DNA in RAT4 cells and virus rescue on infection with Friend murine leukemia virus yielded MPSV which transformed fibroblasts in vitro and also induced spleen foci in adult mice, thus proving that both properties are coded by the same viral genome.