Tumor-specific Polyadenylated RNA's from 7,12-Dimethylbenz(a)anthracene-induced Mammary Tumors Revealed through Hybridization with Fractionated Single-Copy DNA

Polyadenylated (poly(A)+) and nonpolyadenylated (poly(A)-) RNA fractions from primary, hormone-dependent, 7,12-dimethylbenz(a)anthracene-induced mammary tumors and normal rat mammary tissue were analyzed by molecular hybridization to determine if there is a unique class of “tissue-specific” RNA sequences. Single-copy [3H]DNA was fractionated into probes which were either depleted of midpregnant RNA sequences or complementary to these same sequences. When these probes were annealed to homologous and heterologous poly(A)+ RNA's, distinct sets of single-copy sequences were found in the two poly(A)+ RNA populations. The complexity of the “tumor-specific” sequences was about 6.7 × 107 nucleotides (assuming asymmetrical transcription). Hybridization of the poly(A)− RNA fractions isolated from the 7,12-dimethylbenz(a)anthracene-induced tumors and the normal mammary gland to poly(A)− complementary DNA's revealed that both tissues appear to contain a class of poly(A)− RNA sequences which were distinct from the set of poly(A)+ RNA's. Both poly(A)− RNA fractions saturated at 3% when hybridized to single-copy [3H]DNA. However, approximately 15 to 25% (minimum estimate) of the complexity of the poly(A)− RNA was contributed by contaminating poly(A)+ RNA sequences. Therefore, the actual complexity of the poly(A)− RNA fractions was 2.0 to 2.5% of the single-copy hybrid genome. Mixtures of these two RNA fractions also gave a saturation value of 3.0% when annealed to single-copy [3]DNA. This indicated that most of the poly(A)− RNA's were similar in the neoplastic and normal mammary tissues. Although the majority of the infrequent RNA species are held in common between the two tissues, there is a distinct set of high-complexity poly(A)+ RNA sequences, presumably of nuclear origin, which is unique to the 7,12-dimethylbenz(a)anthracene-induced tumors. Whether these RNA's are processed and became functional messenger RNA's is not known. Presumably, they play a role in regulating the frequencies of the tissue-specific abundant and moderately abundant messenger RNA's and therefore provide one mechanism by which the concentration of specific proteins and ultimately the expression of the transformed phenotype may be regulated.