Most highly repeated dispersed DNA families in the mouse genome

The construction of a small library of mouse repetitive DNA has been previously reported (Pietras et al., Nucleic Acids Res. 11:6965-6983, 1983). Here we report that the 35 plasmids in this library corresponding to highly repeated (greater than 30,000 copies per genome) dispersed DNA sequences can be grouped into no more than 5 distinct families. These families together comprise 8 to 10% of the mouse genome. They include the previously described small elements B1, B2, and R and the large MIF-1 element. Twelve of the 35 clones contain evolutionarily conserved (EC) sequences. One EC clone in our library mostly consists of alternating dCdT residues; another consists of tandem repeats of the sequence CCTCT. The majority of B1s and B2s in the genome appear to be homogeneous, whereas R sequences, ECs, and MIF-1s are heterogeneous. Two earlier reports showed highly repeated mammalian DNA sequences in the herpesvirus genome (Peden et al., Cell 31:71-80, 1982; Puga et al., Cell 31:81-87, 1982). We show that sequences homologous to our EC clones are present in the herpesvirus genome, although these polypyrimidine stretches are not detected in poxvirus, adenovirus, and simian virus 40 genomes. We detect transcripts containing homology to all of these sequences in a nuclear transcription assay. Also, we show that small, polyadenylated RNA molecules homologous to B2 sequences are expressed in undifferentiated embryonal carcinoma cells but not in their differentiated derivatives. The significance of these findings is discussed.