Karpen G H, Schaefer J E, Laird C D
Department of Genetics, University of Washington, Seattle 98195.
Genes Dev. 1988 Dec;2(12B):1745-63. doi: 10.1101/gad.2.12b.1745.
P-element transformants of a single rRNA gene (rDNA) were used to investigate the relationship between the organization of the nucleolus organizer (NO) and rDNA function in Drosophila melanogaster. In situ hybridization to rRNA in polytene nuclei of salivary glands demonstrated that an rRNA gene can be transcribed at a high rate when inserted into chromosomal sites other than the NO. Structures that resemble morphologically the endogenous nucleoli ('mininucleoli') were associated with four different euchromatic sites of rDNA insertion. Molecular analyses revealed that these mininucleoli contained both rRNA and an antigen specific to nucleoli. Phenotypes resulting from rDNA deficiencies were rescued partially by the presence of the transformed rDNA, indicating that the transcripts and mininucleoli associated with the rDNA insertion sites were functional. Thus, two conserved features of rDNA organization in eukaryotes, namely tandem repetition and heterochromatic localization, are not required for rRNA gene function. We conclude that 'nucleolar organizing activity' is an intrinsic property of the rDNA or its RNA products.
利用单个核糖体RNA基因（rDNA）的P因子转化体来研究果蝇核仁组织区（NO）的组织与rDNA功能之间的关系。对唾液腺多线核中的rRNA进行原位杂交表明，当rRNA基因插入到NO以外的染色体位点时，它能够以高速率转录。在形态上类似于内源性核仁的结构（“微型核仁”）与rDNA插入的四个不同常染色质位点相关。分子分析显示，这些微型核仁既包含rRNA，也包含一种核仁特异性抗原。rDNA缺陷导致的表型因转化的rDNA的存在而得到部分挽救，这表明与rDNA插入位点相关的转录本和微型核仁具有功能。因此，真核生物中rDNA组织的两个保守特征，即串联重复和异染色质定位，对于rRNA基因功能而言并非必需。我们得出结论，“核仁组织活性”是rDNA或其RNA产物的一种内在特性。
