Identification of a unique mouse mammary tumor virus in the BALB/cNIV mouse strain

We examined the genetic structure, in terms of restriction endonuclease recognition sites, of the milk-transmitted, low-oncogenic mouse mammary tumor virus (MuMTV) of the BALB/cNIV mouse strain. An analysis with EcoRI documented the presence of acquired cNIV proviruses in the mammary tumor DNAs of BALB/cNIV animals. A comparison of tumor DNAs digested with PstI showed that both the cNIV MuMTV and C3Hf MuMTV proviruses lacked the 4.3- and 1.1-kilobase pair fragments characteristic of C3H MuMTV patterns. An examination of mammary tumor and normal, nonmammary tissue DNAs with BamHI supported the idea that the cNIV MuMTV is identical to the C3Hf MuMTV and demonstrated that these two low-oncogenic proviruses are identical to the high-oncogenic C3H MuMTV provirus with respect to a pair of BamHI sites which define a 1.3-kilobase pair fragment. For each of the three MuMTV strains, we also mapped DNAs generated in isolated virions by reverse transcription of their genomic RNAs. Our results showed that cNIV and C3Hf MuMTV are distinct entities by virtue of an additional PstI site within the cNIV long terminal repeat sequence. Another unique feature of cNIV MuMTV revealed by the analysis of virion-generated DNAs was the existence of a family of genomes within the cNIV population. We concluded that cNIV is distinct from its presumptive C3Hf MuMTV predecessor.