Intracytoplasmic A particles can be found in the cytoplasm of mammary tumours, certain leukaemias and Leydig cell tumours of the mouse. These intracytoplasmic A particles share antigenic determinants with the mouse mammary tumour virus (MTV) and are considered to be precursors of MTV B-type particles on the basis of morphological comparison. Both DNA and RNA have been reported to be associated with these intracytoplasmic A particles (Tanaka, Tamura & Tsujimura, 1972; Smith, Litwach & Longfellow, 1974). In this study it is shown that (1) the RNA of intracytoplasmic A particles is homologous to the RNA of MTV; (2) the DNA of the intracytoplasmic A particles is not a replicative form of the MTV genome, because no hybridization was observed between this intracytoplasmic A particle DNA and the radioactively labelled RNA or cDNA of MTV; (3) the intracytoplasmic A particles also contain an RNA-directed DNA polymerase activity with a cation preference which is similar to that of the reverse transcriptase of the MTV in a poly(rC).oligo(dG)-directed DNA polymerase assay.


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