Changes in retrovirus expression in mouse mammary tumor cells in culture.

Mouse mammary tumor cells (Mm5Mt/c1) produced mouse mammary tumor virus (MMTV) (type B retrovirus) at early passages in culture. They apparently produced predominently type C retrovirus(es) at late passages in culture. The decline in MMTV production during passage was not accompanied by an apparent decrease in the synthesis or accumulation of intracellular MMTV-specific RNA. As judged by the concentration of RNA (mol/titer) x half-time (sec) of hybridization of cellular RNA with MMTV complementary DNA, the intracellular concentration MMTV-specific RNA did not change significantly during passage in culture. Since late-passage RNA, particularly polysomal poly(adenylic acid)-containing RNA, yielded lower apparent maximum hybridization of MMTV complementary DNA as compared with early-passage RNA, it was possible that the complexity or base sequence of MMTV-specific RNA in late-passage cultures was not identical with that in early-passage cultures. The data on thermal transitions of hybrids were in accord with this possibility. In contrast, the steady-state level of type C virus-specific RNA in late-passage cultures was orders of magnitude higher than that in early-passage cultures. The production of type C retrovirus(es) at later passages was apparently accompanied by increased transcription of type c retrovirus DNA.