We have used
restriction endonucleases which cleave the
DNA of mouse mammary tumor virus (MMTV) at one site (
Eco RI) and several sites (Pst I, Sac I and Bam HI) to study
infection and mammary
tumorigenesis in mice. Proviruses acquired during
infection of BALB/c mice foster-nursed by virus-producing C3H females can be distinguished from the MMTV proviruses endogenous to uninfected BALB/c mice by the nature of the fragments generated with Pst I and Bam HI. Using this assay, we show that lactating mammary glands as well as mammary
tumors from BALB/cfC3H mice have acquired MMTV
DNA, and that a minimum of approximately 10% of normal glandular cells can be infected. The new proviruses appear to be linked to cellular
DNA of mammary
tumors and infected lactating mammary glands within a limited region (0.2 x 10(6) daltons) of the
viral DNA; the location of this region, based upon mapping studies with unintegrated MMTV
DNA, suggests that the orientation of these proviruses is colinear with linear
DNA synthesized in infected cells and thus approximately colinear with the
viral RNA. Comparisons of many mammary
tumors and studies of lactating mammary glands with a high proportion of independently infected cells indicate that a large number of sites in the cellular genome can accommodate a new provirus; the acquired proviruses are rarely, if ever, found in tandem with each other or with endogenous proviruses. We cannot, however, distinguish between random integration and integration into a large number of preferred sites in the host genome. Since
Eco RI and Bam HI cleavage of
DNA from each mammary
tumor generates a unique set of viral-specific fragments, we propose that the
tumors are composed principally of cells derived from a subset of the many infected cells in a mammary gland; this proposal is supported by our finding that
Eco RI digestion of
DNA from several transplants of a primary
tumor yields the pattern characteristic of the primary
tumor.