Warner J R
Microbiol Rev. 1989 Jun;53(2):256-71. doi: 10.1128/mr.53.2.256-271.1989.
The assembly of a eucaryotic ribosome requires the synthesis of four ribosomal ribonucleic acid (RNA) molecules and more than 75 ribosomal proteins. It utilizes all three RNA polymerases; it requires the cooperation of the nucleus and the cytoplasm, the processing of RNA, and the specific interaction of RNA and protein molecules. It is carried out efficiently and is exquisitely sensitive to the needs of the cell. Our current understanding of this process in the genetically tractable yeast Saccharomyces cerevisiae is reviewed. The ribosomal RNA genes are arranged in a tandem array of 100 to 200 copies. This tandem array has led to unique ways of carrying out a number of functions. Replication is asymmetric and does not initiate from every autonomously replicating sequence. Recombination is suppressed. Transcription of the major ribosomal RNA appears to involve coupling between adjacent transcription units, which are separated by the 5S RNA transcription unit. Genes for many ribosomal proteins have been cloned and sequenced. Few are linked; most are duplicated; most have an intron. There is extensive homology between yeast ribosomal proteins and those of other species. Most, but not all, of the ribosomal protein genes have one or two sites that are essential for their transcription and that bind a common transcription factor. This factor binds also to many other places in the genome, including the telomeres. There is coordinated transcription of the ribosomal protein genes under a variety of conditions. However, the cell seems to possess no mechanism for regulating the transcription of individual ribosomal protein genes in response either to a deficiency or an excess of a particular ribosomal protein. A deficiency causes slow growth. Any excess ribosomal protein is degraded very rapidly, with a half-life of 1 to 5 min. Unlike most types of cells, yeast cells appear not to regulate the translation of ribosomal proteins. However, in the case of ribosomal protein L32, the protein itself causes a feedback inhibition of the splicing of the transcript of its own gene. The synthesis of ribosomes involves a massive transfer of material across the nuclear envelope in both directions. Nuclear localization signals have been identified for at least three ribosomal proteins; they are similar but not identical to those identified for the simian virus 40 T antigen. There is no information about how ribosomal subunits are transported from the nucleus to the cytoplasm.(ABSTRACT TRUNCATED AT 400 WORDS)
真核生物核糖体的组装需要合成四种核糖体核糖核酸（RNA）分子和75种以上的核糖体蛋白。它利用了所有三种RNA聚合酶；需要细胞核和细胞质的协同作用、RNA的加工以及RNA与蛋白质分子的特异性相互作用。该过程高效进行，并且对细胞的需求极为敏感。本文综述了我们目前对在遗传上易于处理的酿酒酵母中这一过程的理解。核糖体RNA基因以100至200个拷贝的串联阵列形式排列。这种串联阵列导致了执行多种功能的独特方式。复制是不对称的，并非从每个自主复制序列起始。重组受到抑制。主要核糖体RNA的转录似乎涉及相邻转录单元之间的偶联，这些转录单元由5S RNA转录单元分隔开。许多核糖体蛋白的基因已被克隆和测序。很少有基因是连锁的；大多数是重复的；大多数含有一个内含子。酵母核糖体蛋白与其他物种的核糖体蛋白之间存在广泛的同源性。大多数（但并非全部）核糖体蛋白基因有一两个对其转录至关重要且能结合一种共同转录因子的位点。该因子也与基因组中的许多其他位置结合，包括端粒。在多种条件下，核糖体蛋白基因会进行协同转录。然而，细胞似乎没有机制来响应特定核糖体蛋白的缺乏或过量而调节单个核糖体蛋白基因的转录。缺乏会导致生长缓慢。任何过量的核糖体蛋白都会迅速降解，半衰期为1至5分钟。与大多数类型的细胞不同，酵母细胞似乎不调节核糖体蛋白的翻译。然而，就核糖体蛋白L32而言，该蛋白本身会对其自身基因转录本的剪接产生反馈抑制。核糖体的合成涉及大量物质在两个方向上穿过核膜的转运。至少已为三种核糖体蛋白鉴定出核定位信号；它们与为猿猴病毒40 T抗原鉴定出的核定位信号相似但不完全相同。关于核糖体亚基如何从细胞核转运到细胞质尚无相关信息。（摘要截取自400字）
