trans-spliceosomal U6 RNAs of Crithidia fasciculata and Leptomonas seymouri: deviation from the conserved ACAGAG sequence and potential base pairing with spliced leader RNA

Unconventional rules of small nuclear RNA transcription and cap modification in trypanosomatids

This review focuses on the spliced leader (SL) RNA and uridylic acid-rich small nuclear RNAs (U-snRNAs) involved in pre-mRNA processing in trypanosomatid protozoa, with particular emphasis on the mechanism of transcription and cap formation. The SL RNA plays a central role in mRNA biogenesis by providing the unique cap 4 structure to the 5' end of all mRNAs by trans-splicing. The trimethylguanosine capped U-snRNAs, on the other hand, represent an unusual example among eukaryotic snRNAs in that they are transcribed by RNA polymerase III. This implies the existence of a distinctive mechanism for capping enzyme selection by the transcriptional machinery. Furthermore, the transcription units of U-snRNA genes offer yet another example of the variety of choices that have been established during eukaryotic evolution, namely that an upstream tRNA gene or tRNA-like gene provides extragenic promoter elements for a downstream small RNA gene.

Spliced Leader RNA-Mediated trans-Splicing in a Dinoflagellate, Karenia brevis

Spliced leader (SL) trans-splicing is a form of mRNA processing originally described in parasitic kinetoplastids. During this reaction, a short RNA sequence is transferred from the 5'-end of an SL transcript to a splice acceptor site on pre-mRNA molecules. Here we report numerous mRNAs from a dinoflagellate, Karenia brevis, which contain an identical leader sequence at their 5'-terminal end. Furthermore, we have isolated a gene from K. brevis encoding a putative SL RNA containing the conserved splice donor site immediately following the leader sequence. A 1,742-bp DNA fragment encoding a K. brevis 5S gene repeat was found to encode the SL RNA gene, as well as a U6 small nuclear RNA (snRNA) gene, and binding sites for the core components of the splicesome (Sm proteins) involved in RNA splicing. Therefore the K. brevis SL RNA appears to be in a genomic arrangement typical of SL genes in a number of species known to mature their mRNAs by trans-splicing. Additionally, we show that the SL gene exists as a stable snRNA and has a predicted secondary structure typical of SL RNAs. The data presented here support the hypothesis that an SL RNA is present in K. brevis and that maturation of a percentage of mRNAs in K. brevis occurs via a trans-splicing process in which a common SL sequence is added to the 5'-end of mature mRNAs. The occurrence of SL trans-splicing in a dinoflagellate extends the known phylogenetic range of this process.

Cloning and molecular characterization of Trypanosoma cruzi U2, U4, U5, and U6 small nuclear RNAs

Small nuclear RNAs (snRNAs) are important factors in the functioning of eukaryotic cells that form several small complexes with proteins; these ribonucleoprotein particles (U snRNPs) have an essential role in the pre-mRNA processing, particularly in splicing, catalyzed by spliceosomes, large RNA-protein complexes composed of various snRNPs. Even though they are well defined in mammals, snRNPs are still not totally characterized in certain trypanosomatids as Trypanosoma cruzi. For this reason we subjected snRNAs (U2, U4, U5, and U6) from T. cruzi epimastigotes to molecular characterization by polymerase chain reaction (PCR) and reverse transcription-PCR. These amplified sequences were cloned, sequenced, and compared with those other of trypanosomatids. Among these snRNAs, U5 was less conserved and U6 the most conserved. Their respective secondary structures were predicted and compared with known T. brucei structures. In addition, the copy number of each snRNA in the T. cruzi genome was characterized by Southern blotting.

Functional analyses of positions across the 5' splice site of the trypanosomatid spliced leader RNA. Implications for base-pair interaction with U5 and U6 snRNAs

In this study, we have used a genetic compensatory approach to examine the functional significance of the previously proposed interaction of spliced leader (SL) RNA with U5 small nuclear RNA (snRNA) (Dungan, J. D., Watkins, K. P., and Agabian, N. (1996) EMBO J. 15, 4016-4029; Xu, Y.-X., Ben Shlomo, H., and Michaeli, S. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 8473-8478) and the interaction of the SL RNA intron with U6 snRNA analogous to cis-splicing. Mutations were introduced at positions -4, -1, +1, +4, +5, and +7/+8 relative to the SL RNA 5' splice site that were proposed to interact with U5 and U6 snRNAs. All mutants exhibited altered splicing phenotypes compared with the parental strain, showing the importance of these intron and exon positions for trans-splicing. Surprisingly, mutation at invariant +1 position did not abolish splicing completely, unlike cis-splicing, but position +2 had the most severe effect on trans-splicing. Compensatory mutations were introduced in U5 and U6 snRNAs to examine whether the defects resulted from failure to interact with these snRNAs by base pairing. Suppression was observed only for positions +5 and +7/+8 with U5 compensatory mutations and for position +5 with a U6 compensatory mutation, supporting the existence of a base pair interaction of U5 and U6 with the SL RNA intron region. The failure to suppress the other SL RNA mutants by the U5 compensatory mutations suggests that another factor(s) interacts with these key SL RNA positions.

U4 small nuclear RNA genes of trypanosomes: cloning of the Leptomonas seymouri gene and mutational analysis of core snRNP assembly

Trans mRNA splicing in trypanosomatids requires as cofactors small nuclear RNAs (snRNAs) U2, U4, U5, and U6, in addition to the spliced leader (SL) RNA. To allow a phylogenetic comparison and functional analysis of trypanosomatid U4 snRNAs, we have cloned the single-copy gene for the Leptomonas seymouri U4 snRNA. In addition, a putative U4 snRNA gene from Leishmania tarentolae was identified by database searching. Using an episomal expression system, we introduced mutations into the conserved Sm region of the L. seymouri U4, which is the putative binding site for the common proteins that are present in each of the trans-spliceosomal snRNPs. As demonstrated by CsCl density gradient centrifugation, Sm mutant U4 snRNAs are non-functional in core RNP assembly. Furthermore, we present evidence by cell fractionation that U4 snRNAs with Sm mutations are partially defective in nuclear-cytoplasmic translocation. Taken together this indicates that the Sm site of U4 snRNA is responsible for stable core RNP assembly and nuclear localization.

Structural conservation and variation among U5 small nuclear RNAs from trypanosomatid protozoa

U5 snRNAs in trypanosomatid protozoa do not contain the trimethylguanosine cap structures that are often targeted in snRNA isolation procedures. As a result, the trypanosomatids are not well represented in the database of available U5 snRNA sequences. We have isolated and determined the sequence of the U5 snRNA from Crithidia fasciculata. Comparison with previously published trypanosomatid U5 snRNA sequences allows us to deduce the pattern of structural conservation and variation among these very divergent snRNA molecules.

The trans-spliceosomal U4 RNA from the monogenetic trypanosomatid Leptomonas collosoma. Cloning and identification of a transcribed trna-like element that controls its expression

U4 small nuclear RNA is essential for trans-splicing. Here we report the cloning of U4 snRNA gene from Leptomonas collosoma and analysis of elements controlling its expression. The trypanosome U4 RNA is the smallest known, it carries an Sm-like site, and has the potential for extensive intermolecular base pairing with the U6 RNA. Sequence analysis of the U4 locus indicates the presence of a tRNA-like element 86 base pairs upstream of the gene that is divergently transcribed to yield a stable small tRNA-like RNA. Two additional tRNA genes, tRNA(Pro) and tRNA(Gly), were found upstream of this element. By stable expression of a tagged U4 RNA, we demonstrate that the tRNA-like gene, but not the upstream tRNA genes, is essential for U4 expression and that the B box but not the A Box of the tRNA-like gene is crucial for expression in vivo. Mapping the 2'-O-methyl groups on U4 and U6 small nuclear RNAs suggests the presence of modifications in canonical positions. However, the number of modified nucleotides is fewer than in mammalian homologues. The U4 genomic organization including both tRNA-like and tRNA genes may represent a relic whereby trypanosomatids "hired" tRNA genes to provide extragenic promoter elements. The close proximity of tRNA genes to the tRNA-like molecule in the U4 locus further suggests that the tRNA-like gene may have evolved from a tRNA member of this cluster.

A candidate U1 small nuclear RNA for trypanosomatid protozoa

In trypanosomatid protozoa, all mRNAs obtain identical 5'-ends by trans-splicing of the 5'-terminal 39 nucleotides of a small spliced leader RNA to appropriate acceptor sites in pre-mRNA. Although this process involves spliceosomal small nuclear (sn) RNAs, it is thought that trypanosomatids do not contain a homolog of the cis-spliceosomal U1 snRNA. We show here that a trypanosomatid protozoon, Crithidia fasciculata, contains a novel small RNA that displays several features characteristic of a U1 snRNA, including (i) a methylguanosine cap and additional 5'-terminal modifications, (ii) a potential binding site for common core proteins that are present in other trans-spliceosomal ribonucleoproteins, (iii) a U1-like 5'-terminal sequence, and (iv) a U1-like stem/loop I structure. Because trypanosomatid pre-mRNAs do not appear to contain cis-spliced introns, we argue that this previously unrecognized RNA species is a good candidate to be a trans-spliceosomal U1 snRNA.

The role of intron structures in trans-splicing and cap 4 formation for the Leishmania spliced leader RNA

A 39-nucleotide leader is trans-spliced onto all trypanosome nuclear mRNAs. The precursor spliced leader RNA was tested for trans-splicing function in vivo by mutating the intron. We report that in Leishmania tarentolae spliced leader RNA 5' modification is influenced by the primary sequence of stem-loop II, the Sm-binding site, and the secondary structure of stem-loop III. The sequence of stem-loop II was found to be important for cap 4 formation and splicing. As in Ascaris, mutagenesis of the bulge nucleotide in stem-loop II was detrimental to trans-splicing. Because restoration of the L. tarentolae stem-loop II structure was not sufficient to restore splicing, this result contrasts the findings in the kinetoplastid Leptomonas, where mutations that restored stem-loop II structure supported splicing. Methylation of the cap 4 structure and splicing was also dependent on both the Sm-binding site and the structure of stem-loop III and was inhibited by incomplete 3' end processing. The critical nature of the L. tarentolae Sm-binding site is consistent with its essential role in the Ascaris spliced leader RNA, whereas in Leptomonas mutation of the Sm-binding site and deletion of stem-loop III did not affect trans-splicing. A pathway for Leishmania spliced leader RNA processing and maturation is proposed.

Regulated tRNA import in Leishmania mitochondria

The genes for three new tRNA and a 5S RNA were identified from a genomic DNA clone of 917 nucleotide pairs from the protozoon Leishmania tarentolae. They were encoded in the following order. The transcriptional directions and anticodons are in parentheses: tRNA(Val) (CAC-->)-5SRNA (-->)-tRNA(His) (<--GUG)-tRNA(Phe) (GAA-->). The tRNA(His) and tRNA(Phe) sequences have not been reported previously in trypanosomatid organisms. By northern analysis, tRNA(Val) and tRNA(Phe) were equally distributed between the cytosol and mitochondria, while tRNA(His) was less abundant in mitochondria than in the cytosol. Accordingly, the latter tRNA is classified as Import restricted (Impr). As shown before, 5S RNA was not imported. Recently, Mahapatra and Adhya [S. Mahapatra, T. Ghosh, S. Adhya, Nucl. Acids Res. 22 (1994) 3381-3386; S. Mahapatra, S. Adhya, J. Biol. Chem. 271 (1996) 20432-20437] have developed an in vitro import system in Leishmania and suggested that the D-loop sequence could serve as the import determinant. We examined all available tRNA gene sequences in trypanosomatids but found no apparent consensus within the D-loop that might account for tRNA-import regulation.

Identification and analysis of the u6 small nuclear RNA gene from Entamoeba histolytica

Among the small nuclear RNAs (snRNAs) involved in the spliceosomal processing of pre-mRNA, U6 is the most conserved. As a first evidence for the presence of the splicing machinery in the amitochondrial protozoan Entamoeba histolytica (Eh), we have cloned the u6 snRNA gene. We find that in this organism u6 is a single copy gene that is transcribed as a poly(A)- RNA molecule of approximately 105 nucleotides. We have mapped the 5' end of the U6 snRNA transcript, and identified typical elements of a putative polymerase III promoter. This is the first snRNA gene reported in Eh. Sequence analysis indicates that this gene contains all the conserved nucleotides known to be important for U6 snRNA function. These results, in conjunction with the earlier finding of genes that contain pre-mRNA introns, suggest that Eh has a functional spliceosomal complex.

Spliced leader RNA of trypanosomes: in vivo mutational analysis reveals extensive and distinct requirements for trans splicing and cap4 formation

In trypanosomes mRNAs are generated through trans splicing. The spliced leader (SL) RNA, which donates the 5'-terminal mini-exon to each of the protein coding exons, plays a central role in the trans splicing process. We have established in vivo assays to study in detail trans splicing, cap4 modification, and RNP assembly of the SL RNA in the trypanosomatid species Leptomonas seymouri. First, we found that extensive sequences within the mini-exon are required for SL RNA function in vivo, although a conserved length of 39 nt is not essential. In contrast, the intron sequence appears to be surprisingly tolerant to mutation; only the stem-loop II structure is indispensable. The asymmetry of the sequence requirements in the stem I region suggests that this domain may exist in different functional conformations. Second, distinct mini-exon sequences outside the modification site are important for efficient cap4 formation. Third, all SL RNA mutations tested allowed core RNP assembly, suggesting flexible requirements for core protein binding. In sum, the results of our mutational analysis provide evidence for a discrete domain structure of the SL RNA and help to explain the strong phylogenetic conservation of the mini-exon sequence and of the overall SL RNA secondary structure; they also suggest that there may be certain differences between trans splicing in nematodes and trypanosomes. This approach provides a basis for studying RNA-RNA interactions in the trans spliceosome.

Structure of the Leptomonas seymouri trans-spliceosomal U2 snRNA-encoding gene; potential U2-U6 snRNA interactions conform to the cis-splicing counterpart

We have characterized the U2 small nuclear RNA (snRNA)-encoding gene from the monogenetic trypanosomatid, Leptomonas seymouri (Ls), to begin to identify the RNA-RNA interactions that direct trans-splicing in kinetoplastid protozoa. The U2 gene, which is single copy in this organism, was isolated and sequenced. Although the Ls U2 snRNA contains many of the sequence and secondary structure elements that are conserved among the U2 snRNAs of cis-splicing organisms, it lacks the stem-loop III region and the intron branch point-recognition region, as do other trypanosomatid U2 snRNAs. A transcriptional promoter element within the Trypanosoma brucei U2 gene [Fantoni et al., Mol. Cell. Biol. 14 (1994) 2021-2028] is conserved in the homologous Ls gene. A crucial step in cis-splicing reactions involves specific base-pairing interactions between the U2 and U6 snRNAs. We show here that in trypanosomatids, where no cis-splicing occurs, these same interactions are possible. This highlights key similarities between the two RNA processing events.

Unexpected diversity in U6 snRNA sequences from trypanosomatids

We have isolated and sequenced the genes for the trans-spliceosomal U6 small nuclear RNAs (snRNAs) from the trypanosomatid species Leishmania mexicana (Lm) and Phytomonas sp. (Ps). Compared with the Trypanosoma brucei (Tb) U6 snRNA, the Lm U6 snRNA contains only a single additional G-C bp in the 5' terminal stem-loop. In contrast, the Ps U6 snRNA sequence contains a G-->C change at the last nucleotide of the highly conserved and functionally important ACAGAG hexanucleotide and three additional changes in conserved positions. Our results indicate that trans-spliceosomal U6 snRNAs from trypanosomatid species do not always conform to the consensus sequence of cis-spliceosomal U6 snRNAs.

trans-splicing: an update

5'-end maturation of messenger RNAs via acquisition of a trans-spliced leader sequence occurs in several primitive eukaryotes, some of which are parasitic. This type of trans-splicing proceeds though a two-step reaction pathway directly analogous to that of cis-splicing and like cis-splicing it requires multiple U snRNP cofactors. This minireview attempts to provide a brief synopsis of our current understanding of the evolution and biological significance of trans-splicing. Progress in deciphering the mechanism of trans-splicing, particularly as it relates to current models of cis-splicing, is also discussed.

In vivo structural analysis of spliced leader RNAs in Trypanosoma brucei and Leptomonas collosoma: a flexible structure that is independent of cap4 methylations

The formation of the mRNA 5' end in trypanosomatid protozoa is carried out by trans-splicing, which transfers a spliced leader (SL) sequence and its hypermethylated cap (cap4) from the SL RNA to the pre-mRNA. Previous in vitro studies with synthetic uncapped RNAs have shown that the SL sequence of Leptomonas collosoma can assume two alternate conformations, Form 1 and Form 2, with Form 1 being the dominant one. To gain information about the structure of the SL RNA in vivo, in its protein-rich environment, we have used permeable Trypanosoma brucei and L. collosoma cells for chemical modification experiments. We introduce the use in vivo of the water-soluble reagents CMCT and kethoxal. In contrast to the in vitro results, the Form 2 secondary structure predominates. However, there are chemically accessible regions that suggest conformational flexibility in SL RNPs and a chemically inaccessible region suggestive of protection by protein or involvement in tertiary interactions. Using complementary 2'-O-methyl RNA oligonucleotides, we show that T. brucei SL RNA can be induced to switch conformation in vivo. SL RNA stripped of proteins and probed in vitro does not display the same Form 2 bias, indicating that SL RNA structure is determined, at least in part, by its RNP context. Finally, the methyl groups of the cap4 do not seem to affect the secondary structure of T. brucei SL RNA, as shown by chemical modification of undermethylated SL RNA probed in vivo.

The U6 snRNA-encoding gene of the monogenetic trypanosomatid Leptomonas collosoma

The U6 snRNA (U6) is the most conserved small nuclear RNA (snRNA) and apparently plays a central role in catalysis of the cis-splicing reaction. In trans-splicing, U6 may have an additional function. In the nematode trans-splicing system, a direct interaction between the U6 and spliced leader (SL) RNAs has been demonstrated, suggesting that U6 may serve as a bridge between the SL RNA and the acceptor pre-mRNA. To examine possible phylogenetic conservation of trypanosomatid U6 sequences that may interact with spliceosomal RNAs, we have cloned and sequenced the U6 gene from the monogenetic trypanosomatid Leptomonas collosoma (Lc). The Lc U6 deviates from the Trypanosoma brucei (Tb) RNA only in four positions located in the 5' stem-loop and the central domains. As in Tb, U6 is a single-copy gene and two tRNA genes, tRNAGln and tRNAIle, are found upstream to the gene. The tRNAs are differentially expressed; tRNAGln is transcribed in the opposite direction to U6, whereas tRNAIle is not transcribed. Possible base-pairing between U6 and the U2 and SL RNAs, similar to the interactions that take place in the nematode trans-splicing system, are proposed.