Gene order of murine mammary tumor virus gag proteins and env proteins

The gene orders of murine mammary tumor virus (MuMTV) gag proteins and env proteins have been determined by pactamycin mapping techniques. A MuMTV producing cell line, Mm5mt, was pulse-labeled with [³¹S]methionine in the presence or absence of 5 × 10^?7 M pactamycin, an inhibitor of initiation of protein synthesis. Both pactamycin and the labeled amino acid were removed after the pulsing period and cells were further incubated in normal growth medium. Virus was harvested from the medium after 24 hr, and the individual protein constituents of the virus were analyzed by SDS-gel electrophoresis. For each protein, the ratio of the radioactivity incorporation in the presence of pactamycin and in its absence was determined. The lower this ratio, the closer would be the protein to the amino terminal of the precursor polyprotein. The following gene orders were derived from these experiments: gag, NH, p10(p28,pp23)p14 COOH, and env, NH₂ gp52gp36 COOH.