The U5 RNA of trypanosomes deviates from the canonical U5 RNA: The Leptomonas collosoma U5 RNA and its coding gene

Fractionation of the abundant small ribonucleoproteins (RNPs) of the trypanosomatid Leptomonas collosoma revealed the existence of a group of unidentified small RNPs that were shown to fractionate differently than the well-characterized trans-spliceosomal RNPs. One of these RNAs, an 80-nt RNA, did not possess a trimethylguanosine (TMG) cap structure but did possess a 5′ phosphate terminus and an invariant consensus U5 snRNA loop 1. The gene coding for the RNA was cloned, and the coding region showed 55% sequence identity to the recently described U5 homologue of Trypanosoma brucei [Dungan, J. D., Watkins, K. P. & Agabian, N. (1996) EMBO J. 15, 4016–4029]. The L. collosoma U5 homologue exists in multiple forms of RNP complexes, a 10S monoparticle, and two subgroups of 18S particles that either contain or lack the U4 and U6 small nuclear RNAs, suggesting the existence of a U4/U6⋅U5 tri-small nuclear RNP complex. In contrast to T. brucei U5 RNA (62 nt), the L. collosoma homologue is longer (80 nt) and possesses a second stem–loop. Like the trypanosome U3, U6, and 7SL RNA genes, a tRNA gene coding for tRNA(Cys) was found 98 nt upstream to the U5 gene. A potential for base pair interaction between U5 and SL RNA in the 5′ splice site region (positions −1 and +1) and downstream from it is proposed. The presence of a U5-like RNA in trypanosomes suggests that the most essential small nuclear RNPs are ubiquitous for both cis- and trans-splicing, yet even among the trypanosomatids the U5 RNA is highly divergent.

对锥虫科聚团细滴虫（Leptomonas collosoma）中丰富的小核糖核蛋白（small ribonucleoproteins, RNPs）进行分级分离分析后发现，存在一类未被鉴定的小核糖核蛋白，其分级分离行为与已被充分表征的反式剪接体小核糖核蛋白（trans-spliceosomal RNPs）存在显著差异。其中一种RNA长度为80nt，不具备三甲基鸟苷（trimethylguanosine, TMG）帽结构，但带有5′磷酸末端，且包含保守恒定的U5小核RNA（U5 small nuclear RNA, U5 snRNA）环1序列。编码该RNA的基因已被克隆，其编码区与此前报道的布氏锥虫（Trypanosoma brucei）U5同源物的序列同一性为55%[Dungan, J. D., Watkins, K. P. & Agabian, N. (1996) EMBO J. 15, 4016–4029]。该U5同源物以多种核糖核蛋白复合物形式存在：包括10S单颗粒，以及两类18S颗粒亚群——分别携带或不携带U4和U6小核RNA，这提示U4/U6⋅U5三小核核糖核蛋白复合物（U4/U6⋅U5 tri-small nuclear RNP complex）的存在。与布氏锥虫的U5 RNA（长度62nt）不同，聚团细滴虫的U5同源物更长（80nt），且带有第二个茎环结构。与锥虫的U3、U6及7SL RNA基因类似，在该U5基因上游98nt处，发现了编码半胱氨酸转运RNA（tRNA(Cys)）的tRNA基因。本研究提出，U5 RNA与剪接前导RNA（spliced leader RNA, SL RNA）在5′剪接位点区域（位置-1与+1）及其下游存在碱基配对相互作用的潜力。锥虫体内存在类U5 RNA这一现象表明，对顺式剪接与反式剪接而言，最核心的小核核糖核蛋白均普遍存在；但即便在锥虫科物种中，U5 RNA也呈现出高度的序列趋异性。