Maden B E, Dent C L, Farrell T E, Garde J, McCallum F S, Wakeman J A
Department of Biochemistry, University of Liverpool, U.K.
Biochem J. 1987 Sep 1;246(2):519-27. doi: 10.1042/bj2460519.
We have isolated several new clones of human ribosomal DNA. Each clone contains part of the external transcribed spacer, a complete 18 S-rRNA gene, the internal transcribed spacers, a complete 28 S-rRNA gene and a short downstream flanking region. We present a detailed map of the human ribosomal transcription unit with the locations of numerous useful restriction sites. In particular, a unique NheI site in the 5.8 S-rRNA gene enabled this gene to be mapped with respect to the 18 S-rRNA and 28 S-rRNA genes. The human 45 S-rRNA coding region is approx. 13,000 nucleotide residues long, of which the external transcribed spacer comprises approx. 3700 nucleotide residues and the first and second internal transcribed spacers comprise approx. 1070 and 1200 nucleotide residues respectively. A partial survey for sites of variation between clones has revealed a single point of variation among 18 S-rRNA gene sequences (a T/C variation at position 140), several sites of length variation in the regions of the transcribed spacers closely flanking the 18 S-rRNA genes, and some sites of length variation among 28 S-rRNA genes. Most of these sites of variation are associated with simple sequence tracts and are in regions that are known to undergo relatively rapid evolutionary divergence. In particular, the sites of variation among 28 S-rRNA genes occur in G + C-rich tracts whose lengths vary among vertebrates and that can be correlated with extensive hairpin structures previously observed by electron microscopy. Each of the clones so far surveyed in detail differs from the others in one or more respects.
我们分离出了几个新的人类核糖体DNA克隆。每个克隆都包含部分外部转录间隔区、一个完整的18 S - rRNA基因、内部转录间隔区、一个完整的28 S - rRNA基因以及一段短的下游侧翼区域。我们展示了人类核糖体转录单元的详细图谱，其中标注了众多有用的限制性酶切位点的位置。特别地，5.8 S - rRNA基因中的一个独特的NheI位点使得该基因能够相对于18 S - rRNA和28 S - rRNA基因进行定位。人类45 S - rRNA编码区约13000个核苷酸残基长，其中外部转录间隔区约占3700个核苷酸残基，第一和第二内部转录间隔区分别约占1070和1200个核苷酸残基。对克隆间变异位点的部分调查揭示了18 S - rRNA基因序列中有一个变异点（第140位的T/C变异），18 S - rRNA基因紧邻的转录间隔区区域有几个长度变异位点，以及28 S - rRNA基因之间的一些长度变异位点。这些变异位点大多与简单序列片段相关，且位于已知经历相对快速进化分歧的区域。特别地，28 S - rRNA基因间的变异位点出现在富含G + C的片段中，这些片段的长度在脊椎动物中有所不同，并且与先前通过电子显微镜观察到的广泛发夹结构相关。到目前为止详细研究的每个克隆在一个或多个方面都与其他克隆不同。
