[Structural organization of the non-transcribed spacer of the rat ribosomal repeated sequence]

Three types of repetitive sequences were found in the non-transcribed spacer of the rat ribosomal genes. Two internal repeats (100-150 bp long) flank the transcribed region encoding pre-rRNA. A number of internal repeats localized at approximately 300 bp upstream the transcription initiation site ranges from 1 to 9. This leads to heterogeneity in this region of the non-transcribed spacer. The non-transcribed spacer comprises of four regions with highly repetitive sequences. Two of these regions are localized 2.0-5.0 kb downstream the 3'-end of the 28S rRNA gene and the other two are localized 3.0-5.0 kb upstream the transcription initiation site. It is shown that these highly repetitive sequences are not only dispersed within the genome, but are found also in the family of extended repeats. One of the dispersed repeats localized upstream the transcribed region is reiterated more than 100 thousand times within the genome and is homologous to the mouse B2 sequence. The dispersed repeats downstream from the transcribed region are repeated 30-40 thousand times within the genome. The nucleotide sequences of the extended region of this sequence (approximately 1.0 kb) has typical blocks of (AC)n, (ACC)n, (GAG)n, (CTGT)n, (TAAC)n, (CCTG)n and (G)n types. Two palindromes and sufficiently long straight repeats were found. The structural-functional organization of the rat ribosomal repeating unit is a convenient model for studying the principles of organization of the genetic material in mammalian cells.