Previous studies have identified human
breast tumor particles possessing many of the features characteristic of RNA tumor viruses. In addition to the expected size (600 S) and density (1.16 g/ml) these include possession of an outer membrane and an inner one surrounding a "core" containing
a DNA polymerase and a large-molecular-weight (70S)
RNA possessing detectable homology to the RNAs of the mouse mammary tumor virus (MMTV) and of the Mason-Pfizer monkey virus (MPMV). We report here the purification and characterization of the
DNA polymerase from the human
breast cancer particles. Its key properties are very similar to those ofthe
RNA-dependent
DNA nucleotidyltransferase (
reverse transcriptase) found in MMTV and MPMV. Thus like these viral
enzymes, the purified human
breast cancer DNA polymerase exhibits the following three features that together distinguish the known viral reverse
transcriptases from normal cellular
DNA polymerases: (i) a strong preference for
oligo(dT)-
poly(rA) over
oligo(dT)-
poly(dA) as a template for the synthesis of
poly(dT); (ii) the acceptance of the highly specific
oligo(dG)-
poly(rCm) as a template for the formation of
poly(dG); (iii) the ability to use a
viral RNA (AMV) as a template to fashion a faithful
DNA complementary copy; and (iv) its preference for Mg++ over Mn++. In summary, the data described here on the
enzyme of the human
breast cancer particles add further evidence of similarities to the viral agents associated with the corresponding
malignancies in the mouse and monkey models. To date, an
enzyme with these properties has not been detected in normal breast tissues or in benign
tumors of the breast.