Lee M G, Van der Ploeg L H
Department of Pathology, New York University, New York 10016, USA.
Annu Rev Microbiol. 1997;51:463-89. doi: 10.1146/annurev.micro.51.1.463.
In eukaryotes, RNA polymerase (pol) II transcribes the protein-coding genes, whereas RNA pol I transcribes the genes that encode the three RNA species of the ribosome [the ribosomal RNAs (rRNAs)] at the nucleolus. Protozoan parasites of the order Kinetoplastida may represent an exception, because pol I can mediate the expression of exogenously introduced protein-coding genes in these single-cell organisms. A unique molecular mechanism, which leads to pre-mRNA maturation by trans-splicing, facilitates pol I-mediated protein-coding gene expression in trypanosomes. Trans-splicing adds a capped 39-nucleotide mini-exon, or spliced leader transcript, to the 5' end of the main coding exon posttranscriptionally. In other eukaryotes, the addition of a 5' cap, which is essential for mRNA function, occurs exclusively as a result of RNA pol II-mediated transcription. Given the assumption that cap addition represents the limiting factor, trans-splicing may have uncoupled the requirement for RNA pol II-mediated mRNA production. A comparison of the alpha-amanitin sensitivity of transcription in naturally occurring trypanosome protein-coding genes reveals that a unique subset of protein-coding genes-the variant surface glycoprotein (VSG) expression sites and the procyclin or the procyclic acidic repetitive protein (PARP) genes-are transcribed by an RNA polymerase that is resistant to the mushroom toxin alpha-amanitin, a characteristic of transcription by RNA pol I. Promoter analysis and a pharmacological characterization of the RNA polymerase that transcribes these genes have strengthened the proposal that the VSG expression sites and the PARP genes represent naturally occurring protein-coding genes that are transcribed by RNA pol I.
在真核生物中，RNA聚合酶（pol）II转录蛋白质编码基因，而RNA pol I在核仁中转录编码核糖体三种RNA种类（核糖体RNA，rRNA）的基因。动质体目的原生动物寄生虫可能是个例外，因为pol I可以介导这些单细胞生物中外源导入的蛋白质编码基因的表达。一种独特的分子机制，通过反式剪接导致前体mRNA成熟，促进了锥虫中pol I介导的蛋白质编码基因表达。反式剪接在转录后将一个带帽的39个核苷酸的小外显子或剪接前导转录本添加到主要编码外显子的5'端。在其他真核生物中，对mRNA功能至关重要的5'帽的添加完全是RNA pol II介导转录的结果。假设帽添加是限制因素，反式剪接可能解除了对RNA pol II介导的mRNA产生的需求。对天然存在于锥虫蛋白质编码基因中的转录对α-鹅膏蕈碱敏感性的比较表明，蛋白质编码基因的一个独特子集——可变表面糖蛋白（VSG）表达位点以及前环蛋白或前环酸性重复蛋白（PARP）基因——由一种对蘑菇毒素α-鹅膏蕈碱具有抗性的RNA聚合酶转录，这是RNA pol I转录的一个特征。对转录这些基因的RNA聚合酶的启动子分析和药理学特征进一步支持了VSG表达位点和PARP基因代表由RNA pol I转录的天然存在的蛋白质编码基因这一观点。
