Nucleotide Sequence Relationship Between Intracisternal Type A Particles of Mus musculus and an Endogenous Retrovirus (M432) of Mus cervicolor

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Abstract
Intracisternal type A particles are retrovirus-like structures found in embryonic cells and many tumors of Mus musculus but having no clear relationship with other retroviruses of this mouse species. We have observed a partial nucleotide sequence homology between the high-molecular-weight (32S and 35S) RNA components of intracisternal A-particles from a neuroblastoma cell line and the 70S RNA fraction from M432, a type of retrovirus endogenous to the Asian mouse Mus cervicolor . M432 complementary DNA (cDNA) was hybridized to the extent of 30% by the A-particle RNAs. The hybrids showed a lower thermal stability (Δ T m , 7°C) than those formed with homologous RNA. The reaction was commensurate with that found between M432 cDNA and divergent sequences in the M. musculus genome. The capacity to hybridize M432 cDNA was closely correlated with the concentration of A-particle sequences in the cytoplasmic RNA of several M. musculus cell types. The major RNA fraction of M432 virus showed a reciprocal partial reaction with the A-particle cDNA's; the virus, which was grown in NIH/3T3 ( M. musculus ) cells, also contained a small proportion of apparently authentic A-particle nucleotide sequences. A subset of A-particle sequences seemed to be almost totally lacking in the main M432 RNA. The A-particle cDNA's hybridized extensively with divergent sequences in M. cervicolor cellular DNA, indicating that this mouse species may contain not only the partially homologous M432 virogene, but also a more complete genetic equivalent of the intracisternal A-particle.