Structural organization of rat ribosomal RNA genes: interspersed sequences and their putative role in the alignment of nucleosomes.

We have observed four regions containing highly repetitive interspersed sequences in the nontranscribed spacer (NTS) of the rat rRNA genes. Two of them (A and B) are located at a distance of 3-5 kb upstream from the transcription start point and two others (C and D) at a distance of 2-5 kb downstream from the 3' end of the 28S rRNA gene. These repetitive sequences are widely dispersed in the genome and are included both in small-copy regions and in the families of extended reiterated sequences. The sequences of three fragments were determined: one from the C2 region, 1100 bp in length and two from A and C1 regions, 110-120 bp long. These regions are characterized by the presence of 'simple' sequences, such as (AC)n, (ACC)n, (GAG)n, (GGGA)n, (TAAG)n, and also of long blocks, (G)n and (A)n. In the C2 region two palindromes, 16 and 14 nt long, were found, one of them including a XhoI site. Mobile element B2 was observed in regions B and C. All four regions, A, B, C and D, contain sets of simple sequences, among which some common elements have been found. Theoretical prediction of the nucleosomal disposition in the C region indicates that the combination of simple sequences existing in the given area secures fixed positions of the nucleosomes, one of the nucleosomes being formed on the B2 element. Moreover, a striking periodicity, with the repeat length close to that of the rat nucleosomal DNA, has been observed. A hypothesis is put forward that the simple sequences can dictate the location of nucleosomes on the adjoining DNA sequences, thereby regulating the gene activity.