Selective expression of cloned middle-repetitive sequences in nuclear RNA of mouse organs

Mouse middle-repetitive sequences have been cloned in the bacterial plasmid pBR322. The DNA sequences, expressed only in nuclei, were screened by filter hybridization using nuclear RNA as probes. Several clones were expressed predominantly in the nuclear RNA in certain types of organs. The copy numbers of typical middle-repetitive sequence clones in genomic DNA were analyzed quantitatively and results showed that 10(3)-10(4) copies exist per haploid DNA, indicating that they belong to a relatively abundant class of repeated sequences. Homology within the family of each middle-repetitive sequence ranged from 81% to 94%. The expression of cloned middle-repetitive sequences was analyzed by liquid-phase hybridization with nuclear RNAs from particular organs. The content of transcripts of the middle-repetitive sequence in nuclear RNA of a particular organ was at least 1000 times higher than that in other organs. In steady-state nuclear RNA these transcripts were calculated to amount to 10(2)-10(4) copies per nucleus. Most of these transcripts seem to be retained in the nuclei and do not appear in the cytoplasm.