Zhang Huan, Hou Yubo, Miranda Lilibeth, Campbell David A, Sturm Nancy R, Gaasterland Terry, Lin Senjie
Department of Marine Sciences, University of Connecticut, 1080 Shennecossett Road, Groton, CT 06340, USA.
Proc Natl Acad Sci U S A. 2007 Mar 13;104(11):4618-23. doi: 10.1073/pnas.0700258104. Epub 2007 Mar 2.
Through the analysis of hundreds of full-length cDNAs from fifteen species representing all major orders of dinoflagellates, we demonstrate that nuclear-encoded mRNAs in all species, from ancestral to derived lineages, are trans-spliced with the addition of the 22-nt conserved spliced leader (SL), DCCGUAGCCAUUUUGGCUCAAG (D = U, A, or G), to the 5' end. SL trans-splicing has been documented in a limited but diverse number of eukaryotes, in which this process makes it possible to translate polycistronically transcribed nuclear genes. In SL trans-splicing, SL-donor transcripts (SL RNAs) contain two functional domains: an exon that provides the SL for mRNA and an intron that contains a spliceosomal (Sm) binding site. In dinoflagellates, SL RNAs are unusually short at 50-60 nt, with a conserved Sm binding motif (AUUUUGG) located in the SL (exon) rather than the intron. The initiation nucleotide is predominantly U or A, an unusual feature that may affect capping, and hence the translation and stability of the recipient mRNA. The core SL element was found in mRNAs coding for a diverse array of proteins. Among the transcripts characterized were three homologs of Sm-complex subunits, indicating that the role of the Sm binding site is conserved, even if the location on the SL is not. Because association with an Sm-complex often signals nuclear import for U-rich small nuclear RNAs, it is unclear how this Sm binding site remains on mature mRNAs without impeding cytosolic localization or translation of the latter. The sequences reported in this paper have been deposited in the GenBank database (accession nos. AF 512889, DQ 864761-DQ 864971, DQ 867053-DQ 867070, DQ 884413-DQ 884451, EF 133854-EF 133905, EF 133961-EF 134003, EF 134083-EF 134402, EF 141835, and EF 143070-EF 143105).
通过对代表甲藻所有主要类目的15个物种的数百个全长cDNA进行分析，我们证明，从原始谱系到衍生谱系的所有物种中，核编码的mRNA都经过反式剪接，在其5'端添加了22个核苷酸的保守剪接前导序列（SL），即DCCGUAGCCAUUUUGGCUCAAG（D = U、A或G）。反式剪接已在数量有限但种类多样的真核生物中得到记载，在这些生物中，这一过程使得多顺反子转录的核基因得以翻译。在反式剪接中，SL供体转录本（SL RNA）包含两个功能域：一个为mRNA提供SL的外显子和一个含有剪接体（Sm）结合位点的内含子。在甲藻中，SL RNA异常短，只有50 - 60个核苷酸，其保守的Sm结合基序（AUUUUGG）位于SL（外显子）而非内含子中。起始核苷酸主要是U或A，这一不寻常的特征可能会影响加帽，进而影响受体mRNA的翻译和稳定性。核心SL元件存在于编码各种蛋白质的mRNA中。在已鉴定的转录本中，有三个是Sm复合亚基的同源物，这表明Sm结合位点的作用是保守的，即使其在SL上的位置并非如此。由于与Sm复合物的结合通常标志着富含U的小核RNA进入细胞核，目前尚不清楚这个Sm结合位点如何保留在成熟mRNA上而不妨碍其胞质定位或翻译。本文报道的序列已存入GenBank数据库（登录号为AF 512889、DQ 864761 - DQ 864971、DQ 867053 - DQ 867070、DQ 884413 - DQ 884451、EF 133854 - EF 133905、EF 133961 - EF 134003、EF 134083 - EF 134402、EF 141835以及EF 143070 - EF 143105）。
