Abstract
Molecular genetic studies of mouse mammary tumor virus (MMTV) have been hampered by the difficulty of cloning proviruses of milk-borne strains because of inhibitory sequences located in the gag gene. To surmount this barrier we have constructed a hybrid MMTV provirus composed of clonable 5' sequences (encompassing gag) from an endogenous MMTV provirus of C3H mice (Mtv-1) and 3' sequences from the milk-borne strain of MMTV in C3H mice, MMTV(C3H). Virions produced from XC cells transfected with this hybrid provirus are infectious in cell culture and tumorigenic in BALB/cJ mice. A vector derived from this provirus, containing the neomycin phosphotransferase gene (neo) and origins of replication from simian virus 40 and pBR322, is capable of transferring G418 resistance by virus infection in cell culture when supplied with viral proteins from either MMTV(C3H) or the hybrid MMTV. Expression of both hybrid and vector proviruses is inducible by dexamethasone in infected cells.
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