[Nuclear and cytoplasmic RNA showing hybridization with repetitive sequences B1 and B2 of the mouse genome].

Heavy nuclear RNA contains high amounts of transcripts from repetitive sequences B1 and B2. Cytoplasmic poly(A+)RNA and in particular, polysomal poly(A)+RNA (mRNA) also contain these sequences though their content is much lower. An abundant 2 kb mRNA of mouse liver was found to contain B2 sequence. A novel class of RNA is described namely as low molecular weight heterogeneous (200-400 nucleotide long) poly(A)+RNA. These RNAs contain B1 and B2 sequences and are located mostly in the cytoplasm but not in polysomes. Tumor cells contain high amounts of small B2+RNAs. The content of B1+ small cytoplasmic RNA is much lower. The hybridization tests confirm the existence of extended homology between B1 sequence and 4.5 S small nuclear RNA which has been earlier deduced from comparison of their base sequences. Homology of B1 and small cytoplasmic 7S RNA was also detected. No hybridization of B2 to small nuclear or small cytoplasmic RNAs could be detected.