Rae P M, Barnett T, Murtif V L
Chromosoma. 1981;82(5):637-55. doi: 10.1007/BF00285773.
Ribosomal DNA nontranscribed spacers in Drosophila virilis DNA have been examined in some detail by restriction site analysis of cloned segments of rDNA, nucleic acid hybridizations involving unfractionated rDNA, and base composition estimates. The overall G + C content of the spacer is 27--28%; this compares with 39% for rDNA as a whole, 40% for main band DNA, and 26% for the D. virilis satellites. Much of the spacer is comprised of 0.25 kb repeats revealed by digestion with Msp I, Fnu DII or Rsd I, which terminate very near the beginning of the template for the ribosomal RNA precursor. The spacers are heterogeneous in length among rDNA repeats, and this is largely accounted for by variation among rDNA units in the number of 0.25 kb elements per spacer. Despite its high A + T content and the repetitive nature of much of the spacer, and the proximity of rDNA and heterochromatin in Drosophila, pyrimidine tract analysis gave no indication of relatedness between the spacer and satellite DNA sequences. Species of Drosophila closely related to D. virilis have rDNA spacers that are homologous with those in D. virilis to the extent that hybridization of a cloned spacer segment of D. virilis rDNA to various DNA is comparable with hybridization to homologous DNA, and distributions of restriction enzyme cleavage sites are very similar (but not identical) among spacers of the various species. There is spacer length heterogeneity in the rDNA of all species, and each species has a unique major rDNA spacer length. Judging from Southern blot hybridization, D. hydei rDNA spacers have 20--30% sequence homology with D. virilis rDNA spacers, and a repetitive component is similarly sensitive to Msp I and Fnu DII digestion. D. melanogaster rDNA spacers have little or no homology with counterparts in D. virilis rDNA, despite a similar content of 0.25 kb repetitive elements. In contrast, sequences in rDNA that encode 18S and 28S ribosomal RNA have been highly conserved during the divergence of Drosophila species; this is inferred from interspecific hybridizations involving ribosomal RNA and a comparison of distributions of restriction enzyme cleavage sites in rDNA.
通过对核糖体DNA（rDNA）克隆片段的限制性酶切位点分析、涉及未分级rDNA的核酸杂交以及碱基组成估计，已对果蝇（Drosophila virilis）DNA中的核糖体DNA非转录间隔区进行了较为详细的研究。间隔区的总体鸟嘌呤加胞嘧啶（G + C）含量为27% - 28%；相比之下，整个rDNA的G + C含量为39%，主带DNA为40%，果蝇卫星DNA为26%。间隔区的大部分由经Msp I、Fnu DII或Rsd I酶切后显示的0.25 kb重复序列组成，这些重复序列在核糖体RNA前体模板起始处附近终止。rDNA重复序列之间的间隔区长度存在异质性，这在很大程度上是由于每个间隔区中0.25 kb元件数量在rDNA单位之间存在差异。尽管间隔区大部分具有高腺嘌呤加胸腺嘧啶（A + T）含量和重复性，且果蝇中rDNA与异染色质相邻，但嘧啶序列分析未显示间隔区与卫星DNA序列之间存在相关性。与果蝇亲缘关系密切的物种具有与果蝇间隔区同源的rDNA间隔区，果蝇rDNA的克隆间隔区片段与各种DNA的杂交情况与与同源DNA的杂交情况相当，并且各种物种间隔区中限制性内切酶切割位点的分布非常相似（但不完全相同）。所有物种的rDNA中都存在间隔区长度异质性，且每个物种都有独特的主要rDNA间隔区长度。从Southern印迹杂交判断，海德氏果蝇（D. hydei）的rDNA间隔区与果蝇的rDNA间隔区有20% - 30%的序列同源性，并且一个重复组分对Msp I和Fnu DII酶切同样敏感。黑腹果蝇（D. melanogaster）的rDNA间隔区与果蝇rDNA中的对应序列几乎没有同源性，尽管它们含有相似含量的0.25 kb重复元件。相比之下，在果蝇物种分化过程中，编码18S和28S核糖体RNA的rDNA序列高度保守；这是通过涉及核糖体RNA的种间杂交以及rDNA中限制性内切酶切割位点分布的比较推断出来的。
