Increased abundance of a normal cell mRNA sequence accompanies the conversion of rat mammary cuboidal epithelial cells to elongated myoepithellal-llke cells in culture

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Abstract
Rat mammary cuboidal epithelial cell lines in culture convert to elongated myoepithelial-like cells. This conversion is accompanied by the appearance of a 9,000 molecular weight acidic polypeptide (p9ka), abundant in the elongated convertants, but which is hardly detectable in the cuboidal epithelial cells. A cDNA library corresponding to a low-molecular-weight fraction of poly(A)- containing RNA from a myoepithelial -like cell line, has been constructed. Recombinant plasmids containing cDNA complementary to p9ka mRNA have been identified by hybrid-selected translation. The mRNA for p9ka has been identified by Northern blotting and is found to be at least five-times more abundant in cultured myoepithelial -like rat mammary cells when compared to the cuboidal epithelial cells. This cytoplasmic mRNA sequence, which 1s present in increased abundance in cultured mammary myoepithelial -like cells, is also present, at lower levels, in normal rat tissues, including the mammary glands.