A new moderately repetitive rat DNA sequence detected by a cloned 4.5 SI DNA.

4.5 SI RNA is an abundant, noncapped, small nuclear RNA found in rodent cells. The 4.5 SI RNA is 98 or 99 nucleotides long and contains no modified nucleotides; it is synthesized by RNA polymerase III, is partly hydrogen-bonded to poly(A+) hnRNA, and was the first small nuclear RNA to be purified and sequenced (Busch, H., Reddy, R., Ruthblum, L., and Choi, Y. C. (1982) Annu. Rev. Biochem. 51, 617-654). In studies on the structure and organization of genes coding for this abundant RNA, it was found that this RNA is homologous to an apparently novel family of repetitive sequences. Two clones were characterized; one clone showed that its sequence is identical to the RNA in the first 92 residues and differed only in the last six nucleotides. In addition, the 3'-end of the sequence contained an A,T-rich region, and the sequence was flanked by a 15-nucleotide long direct repeat of AAAATATAGACACTG. The second clone characterized contained nucleotide sequences 1-57 corresponding to the RNA and was flanked by a 15-nucleotide long direct repeat. The structural features of these two DNAs are consistent with RNA-mediated DNA synthesis and integration of this DNA into the genome at random sites. It is estimated that there are about 10,000 copies of this family of sequences in the haploid rat genome.