Intracisternal a particle-specific DNA sequences in mammary tumor cells, hybrids, and cybrids derived from laboratory mice and from feral mice of Mus musculus and Mus cervicolor

We have examined the DNA sequences homologous to murine intracisternal A-particle (IAP) cDNA by nucleic acid hybridization, using mammary tumor (MT) cells, hybrids and cybrids, and feral mouse cells of Mus musculus and Mus cervicolor. The radioactive cDNA probe was synthesized by the endogenous reverse transcriptase reaction in purified mammary tumor IAP. Given the copy numbers of IAP-specific DNA sequences obtained in the various parent cells, hybrids, and cybrids, and the thermal stability characteristics of the DNA duplexes, two distinct types of phenomena were observed. First, one phenotypically negative recipient cell line, 3T3-4EF, had only 1–2 copies of IAP-specific DNA sequences per haploid genome prior to fusion. When this line was fused to MT cytoplasts (enucleated cells) containing IAP, the resulting cybrid cells were found to have about 10 copies of IAP-specific DNA sequences, which by the Hirt procedure, localized in the detergent-insoluble fraction of the nuclear DNA. Thus the continuous replication of IAP in the MT x 3T3-4EF cybrids was associated with an increase in the number of IAP-specific DNA sequences in the cell genome. Second, another phenotypically negative recipient cell line, INC, had about 4 copies of IAP-specific DNA sequences per haploid genome prior to fusion. Following fusion with MT cytoplasts, the resulting MT x INC cybrids did not show any increase in the number of IAP-specific DNA sequences, even after IAP replication had become overt, continuous, and nonsegregated. The distribution of DNA copies in the various Mus musculus and Mus cervicolor laboratory and feral mouse cells indicates that the IAP genome has been conserved in spite of the divergency of the two species.