Sequence and genetic analyses of the 3 ' terminus and integration sites ofthe RIII/Sa mouse mammary tumor (MMTV) exogenous provirus

The RIII/Sa mouse mammary tumor virus (MMTV) is one of several exogenous MM TV 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 a lso provide a detailed sequence analysis of ten different 3' terminal exoge nous MMTV proviral integration sites within mammary tumor DNA obtained by t he technique of genome walking. Using a combination of Southern blotting wi th 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 mammar y 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 pre viously reported. The combined genetic analyses reported herein distinguish between exogenous proviral integrants and endogenous germline MMTV.