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DNAs extracted from the mammary tumors of GR mice were analyzed for mouse mammary tumor virus proviral sequences by the restriction enzyme-Southern blot procedure. The tumor DNAs contain more proviral copies of mouse mammary tumor virus than DNA from a nonmalignant tissue. The degree of proviral amplification is small (ca. one to five additional copies) and appears to be variable from tumor to tumor. The restriction patterns of the amplified proviral sequences suggest a clonal origin for the tumor mass. In addition, the restriction patterns observed after digestion with the enzymes BglII and SacI indicate that only one of the proviruses endogenous to GR mice is amplified. The amplified provirus found in GR mammary tumors is identical to the provirus that is missing in GR-Mtv-2- mice, a congenic line exhibiting a low mammary tumor incidence.

Mouse mammary tumor virus-producing cultures of mouse mammary tumor cells synthesize a viral-related polypeptide of molecular weight of 73,000 (gp 73) which is rapidly labeled during a short pulse but disappears during the chase concomitantly with the appearance of label in the virion glycoproteins gp 49 and gp 37.5/33.5. The addition of the protein synthesis-inhibitor cycloheximide to the chase medium has little effect on this conversion. Treatment of the proposed precursor with α-chymotrypsin leads to the formation of a polypeptide of molecular weight 49,000, similar to the major virion glycoprotein. A comparison of tryptic digest maps of the glycoproteins involved supports the hypothesis that both the viral glycoproteins gp 49 and gp 37.5/33.5 are derived from gp 73.

Mouse mammary tumor virus polypeptides were detected in the cytoplasm of mouse mammary tumor cell cultures using immunological precipitation techniques. The anti-mouse mammary tumor virus serum precipitated the major virion glycoproteins gp49 and gp37.5/33.5 and a viral-related nonvirion glycoprotein of 76,000 daltons. Subcellular fractionation studies revelaed that the cell-associated virion glycoproteins were present in the membrane fraction. Pulsechase experiments indicated that a viral-related nonvirion glycoprotein of 76,000 daltons may be a precursor to one or more of the virion glycoproteins.