Mair G, Shi H, Li H, Djikeng A, Aviles H O, Bishop J R, Falcone F H, Gavrilescu C, Montgomery J L, Santori M I, Stern L S, Wang Z, Ullu E, Tschudi C
Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520-8022, USA.
RNA. 2000 Feb;6(2):163-9. doi: 10.1017/s135583820099229x.
It has been known for almost a decade and a half that in trypanosomes all mRNAs are trans-spliced by addition to the 5' end of the spliced leader (SL) sequence. During the same time period the conviction developed that classical cis-splicing introns are not present in the trypanosome genome and that the trypanosome gene arrangement is highly compact with small intergenic regions separating one gene from the next. We have now discovered that these tenets are no longer true. Poly(A) polymerase (PAP) genes in Trypanosoma brucei and Trypanosoma cruzi are split by intervening sequences of 653 and 302 nt, respectively. The intervening sequences occur at identical positions in both organisms and obey the GT/AG rule of cis-splicing introns. PAP mRNAs are trans-spliced at the very 5' end as well as internally at the 3' splice site of the intervening sequence. Interestingly, 11 nucleotide positions past the actual 5' splice site are conserved between the T. bruceiand T. cruzi introns. Point mutations in these conserved positions, as well as in the AG dinucleotide of the 3' splice site, abolish intron removal in vivo. Our results, together with the recent discovery of cis-splicing introns in Euglena gracilis, suggest that both trans- and cis-splicing are ancient acquisitions of the eukaryotic cell.
近十五年以来，人们已经知道，在锥虫中，所有的信使核糖核酸（mRNA）都是通过添加到剪接前导序列（SL）的5'端来进行反式剪接的。在同一时期，人们逐渐确信，经典的顺式剪接内含子在锥虫基因组中并不存在，并且锥虫的基因排列非常紧凑，基因之间的间隔区域很小。然而，我们现在发现这些观点不再正确。布氏锥虫和克氏锥虫中的聚腺苷酸聚合酶（PAP）基因分别被653个核苷酸和302个核苷酸的间隔序列所分割。这些间隔序列在两种生物中处于相同的位置，并且遵循顺式剪接内含子的GT/AG规则。PAP信使核糖核酸在5'端以及间隔序列3'剪接位点内部进行反式剪接。有趣的是，布氏锥虫和克氏锥虫内含子在实际5'剪接位点之后的11个核苷酸位置是保守的。这些保守位置以及3'剪接位点的AG二核苷酸中的点突变会消除体内内含子的去除。我们的结果，以及最近在纤细裸藻中发现的顺式剪接内含子，表明反式剪接和顺式剪接都是真核细胞古老的遗传特征。
