Trans-splicing and operons in C. elegans.
Trans-splicing and operons in C. elegans.
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DOI：
10.1895/wormbook.1.5.2
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发表时间：
2012-11-20
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影响因子：
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通讯作者：
Blumenthal, Thomas
中科院分区：
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作者：
Blumenthal, Thomas
约70%的秀丽隐杆线虫mRNA通过反式剪接与两种22个核苷酸的剪接前导序列之一相连。SL1用于修剪前体mRNA的5'端，并将其替换为SL1序列。这种加工事件与顺式剪接（即内含子去除）密切相关。SL1序列由一个100核苷酸的小核核糖核蛋白颗粒（snRNP），即SL1 snRNP提供。这种snRNP在结构和功能上与在去除内含子和反式剪接中起关键作用的U snRNAs（U1、U2、U4、U5和U6）相似，只是SL1 snRNP在该过程中被消耗。超过一半的秀丽隐杆线虫前体mRNA经历SL1反式剪接，而约30%不进行反式剪接。其余基因由SL2进行反式剪接，SL2由一个类似的snRNP，即SL2 snRNP提供。SL2的受体都是紧密排列的基因簇中类似细菌操纵子的下游基因。它们从包含2到8个基因的基因簇5'端的一个启动子转录。这种转录产生一个多顺反子前体mRNA，它在每个基因的3'端通过切割和多聚腺苷酸化进行共转录加工，并且这个事件与仅在下游约100核苷酸处发生的SL2反式剪接事件紧密偶联。SL2反式剪接需要基因间的一个序列，即Ur元件，它可能与SL2 snRNP上的5'剪接位点碱基配对，其方式类似于顺式剪接中5'剪接位点与U1 snRNP之间的相互作用。关键区别在于，在反式剪接中，snRNP包含5'剪接位点，而在顺式剪接中是前体mRNA包含。一些操纵子，被称为“混合操纵子”，在两个基因之间包含一个额外的启动子，它可以表达下游基因，其发育模式与整个操纵子不同。这些操纵子主要包含快速生长所需的基因，包括其产物为线粒体功能和基因表达基本机制所必需的基因。最近的证据表明，RNA聚合酶在生长基因的启动子处处于准备状态，并且操纵子允许从生长停滞状态更有效地恢复，从而减少对这种无活性RNA聚合酶储备的需求。
About 70% of C. elegans mRNAs are trans-spliced to one of two 22 nucleotide spliced leaders. SL1 is used to trim off the 5' ends of pre-mRNAs and replace them with the SL1 sequence. This processing event is very closely related to cis-splicing, or intron removal. The SL1 sequence is donated by a 100 nt small nuclear ribonucleoprotein particle (snRNP), the SL1 snRNP. This snRNP is structurally and functionally similar to the U snRNAs (U1, U2, U4, U5 and U6) that play key roles in intron removal and trans-splicing, except that the SL1 snRNP is consumed in the process. More than half of C. elegans pre-mRNAs are subject to SL1 trans-splicing, whereas ~30% are not trans-spliced. The remaining genes are trans-spliced by SL2, which is donated by a similar snRNP, the SL2 snRNP. SL2 recipients are all downstream genes in closely spaced gene clusters similar to bacterial operons. They are transcribed from a promoter at the 5' end of the cluster of between 2 and 8 genes. This transcription makes a polycistronic pre-mRNA that is co-transcriptionally processed by cleavage and polyadenylation at the 3' end of each gene, and this event is closely coupled to the SL2 trans-splicing event that occurs only ~100 nt further downstream. SL2 trans-splicing requires a sequence between the genes, the Ur element, that likely base pairs with the 5' splice site on the SL2 snRNP, in a manner analogous to the interaction between the 5' splice site in cis-splicing with the U1 snRNP. The key difference is that in trans-splicing, the snRNP contains the 5' splice site, whereas in cis-splicing the pre-mRNA does. Some operons, termed "hybrid operons", contain an additional promoter between two genes that can express the downstream gene or genes with a developmental profile that is different from that of the entire operon. The operons contain primarily genes required for rapid growth, including genes whose products are needed for mitochondrial function and the basic machinery of gene expression. Recent evidence suggests that RNA polymerase is poised at the promoters of growth genes, and operons allow more efficient recovery from growth-arrested states, resulting in reduction in the need for this cache of inactive RNA polymerase.