Singh R, Gupta S, Reddy R
Department of Pharmacology, Baylor College of Medicine, Houston, Texas 77030.
Mol Cell Biol. 1990 Mar;10(3):939-46. doi: 10.1128/mcb.10.3.939-946.1990.
The cap structure of U6 small nuclear RNA (snRNA) is gamma-monomethyl phosphate and is distinct from other known RNA cap structures (R. Singh and R. Reddy, Proc. Natl. Acad. Sci. USA 86:8280-8283, 1989). Here we show that the information for capping the U6 snRNA in vitro is within the initial 25 nucleotides of the U6 RNA. The capping determinant in mammalian U6 snRNA is a bipartite element--a phylogenetically conserved stem-loop structure and an AUAUAC sequence, or a part thereof, following this stem-loop. Wild-type capping efficiency was obtained when the AUAUAC motif immediately followed the stem-loop and when the gamma-phosphate of the initiation nucleotide was in close proximity to the capping determinant. Incorporation of a synthetic stem-loop followed by an AUAUAC sequence is sufficient to covert a noncapped heterologous transcript into a capped transcript. Transcripts with the initial 32 nucleotides of Saccharomyces cerevisiae U6 snRNA are accurately capped in HeLa cell extract, indicating that capping machinery from HeLa cells can cap U6 snRNA from an evolutionarily distant eucaryote. The U6-snRNA-specific capping is unusual in that it is RNA sequence dependent, while the capping of mRNAs and other U snRNAs is tightly coupled to transcription and is independent of the RNA sequence.
U6小核RNA（snRNA）的帽结构是γ-单甲基磷酸酯，与其他已知的RNA帽结构不同（R.辛格和R.雷迪，《美国国家科学院院刊》86:8280 - 8283，1989年）。在此我们表明，体外给U6 snRNA加帽的信息存在于U6 RNA最初的25个核苷酸内。哺乳动物U6 snRNA中的加帽决定因素是一个二分元件——一个系统发育上保守的茎环结构以及该茎环之后的AUAUAC序列或其一部分。当AUAUAC基序紧跟在茎环之后且起始核苷酸的γ-磷酸靠近加帽决定因素时，可获得野生型加帽效率。引入一个合成的茎环后接AUAUAC序列足以将一个未加帽的异源转录本转变为加帽转录本。含有酿酒酵母U6 snRNA最初32个核苷酸的转录本在HeLa细胞提取物中能被准确加帽，这表明HeLa细胞的加帽机制能够给来自进化上距离较远的真核生物的U6 snRNA加帽。U6 - snRNA特异性加帽不同寻常之处在于它依赖于RNA序列，而mRNA和其他U snRNA的加帽与转录紧密偶联且不依赖于RNA序列。
