Sequence and Genetic Analyses of the 3′ Terminus and Integration Sites of the RIII/Sa Mouse Mammary Tumor (MMTV) Exogenous Provirus

The RIII/Sa mouse mammary tumor virus (MMTV) is one of several exogenous MMTV viruses transmitted to suckling mice through the milk. We report herein the nucleotide sequence of the exogenous RIII/Sa provirus from the central Eco RI site through to the end of the U5 region encoded by the 3′ LTR. We also provide a detailed sequence analysis of ten different 3′ terminal exogenous MMTV proviral integration sites within mammary tumor DNA obtained by the technique of genome walking. Using a combination of Southern blotting with 3′ end probes and PCR utilizing a unique RIII/Sa specific 3′ primer, we confirm that the RIII/Sa provirus integrates multiple times in mouse mammary tumors but that little or no integration occurs in various normal tissues. Southern blotting analyses with 3′ end probes also indicate that RIII/Sa mice contain two additional endogenous MMTV loci, mtv-6 and mtv-17, not previously reported. The combined genetic analyses reported herein distinguish between exogenous proviral integrants and endogenous germline MMTV.