The Euglena gracilis rbcS gene contains introns with unusual borders

Unique features of conventional and nonconventional introns in Euglena gracilis

BACKGROUND

Nuclear introns in Euglenida have been understudied. This study aimed to investigate nuclear introns in Euglenida by identifying a large number of introns in Euglena gracilis (E. gracilis), including cis-spliced conventional and nonconventional introns, as well as trans-spliced outrons. We also examined the sequence characteristics of these introns.

RESULTS

A total of 28,337 introns and 11,921 outrons were identified. Conventional and nonconventional introns have distinct splice site features; the former harbour canonical GT/C-AG splice sites, whereas the latter are capable of forming structured motifs with their terminal sequences. We observed that short introns had a preference for canonical GT-AG introns. Notably, conventional introns and outrons in E. gracilis exhibited a distinct cytidine-rich polypyrimidine tract, in contrast to the thymidine-rich tracts observed in other organisms. Furthermore, the SL-RNAs in E. gracilis, as well as in other trans-splicing species, can form a recently discovered motif called the extended U6/5' ss duplex with the respective U6s. We also describe a novel type of alternative splicing pattern in E. gracilis. The tandem repeat sequences of introns in this protist were determined, and their contents were comparable to those in humans.

CONCLUSIONS

Our findings highlight the unique features of E. gracilis introns and provide insights into the splicing mechanism of these introns, as well as the genomics and evolution of Euglenida.

Order of removal of conventional and nonconventional introns from nuclear transcripts of Euglena gracilis

Nuclear genes of euglenids and marine diplonemids harbor atypical, nonconventional introns which are not observed in the genomes of other eukaryotes. Nonconventional introns do not have the conserved borders characteristic for spliceosomal introns or the sequence complementary to U1 snRNA at the 5' end. They form a stable secondary structure bringing together both exon/intron junctions, nevertheless, this conformation does not resemble the form of self-splicing or tRNA introns. In the genes studied so far, frequent nonconventional introns insertions at new positions have been observed, whereas conventional introns have been either found at the conserved positions, or simply lost. In this work, we examined the order of intron removal from Euglena gracilis transcripts of the tubA and gapC genes, which contain two types of introns: nonconventional and spliceosomal. The relative order of intron excision was compared for pairs of introns belonging to different types. Furthermore, intermediate products of splicing were analyzed using the PacBio Next Generation Sequencing system. The analysis led to the main conclusion that nonconventional introns are removed in a rapid way but later than spliceosomal introns. Moreover, the observed accumulation of transcripts with conventional introns removed and nonconventional present may suggest the existence of a time gap between the two types of splicing.

The existence of conventional spliceosomal introns in genes of eukaryotic organisms is a well-known theorem. However, genes of the unicellular algae group, euglenids, contain also another type of introns, so-called nonconventional ones. They lack canonical borders, a feature most characteristic for conventional introns and form a stable secondary structure bringing together their ends. Along with the increasing popularity of whole genome studies, nonconventional introns were also disclosed in the genes of other protists, diplonemids. In this study we were particularly interested which introns–conventional or nonconventional–are removed earlier from euglenids’ pre-mRNA. To track this process we analyzed transcripts of the two Euglena gracilis genes. The relative order of intron excision was compared for pairs of introns belonging to different types. We also surveyed thousands of intermediate products of splicing using the Next-Generation Sequencing system. Summarizing the results of both experiments, we proved that spliceosomal introns are removed at an earlier stage of pre-mRNA maturation than nonconventional ones.

The transcriptome of Euglena gracilis reveals unexpected metabolic capabilities for carbohydrate and natural product biochemistry

Euglena gracilis is a highly complex alga belonging to the green plant line that shows characteristics of both plants and animals, while in evolutionary terms it is most closely related to the protozoan parasites Trypanosoma and Leishmania. This well-studied organism has long been known as a rich source of vitamins A, C and E, as well as amino acids that are essential for the human diet. Here we present de novo transcriptome sequencing and preliminary analysis, providing a basis for the molecular and functional genomics studies that will be required to direct metabolic engineering efforts aimed at enhancing the quality and quantity of high value products from E. gracilis. The transcriptome contains over 30,000 protein-encoding genes, supporting metabolic pathways for lipids, amino acids, carbohydrates and vitamins, along with capabilities for polyketide and non-ribosomal peptide biosynthesis. The metabolic and environmental robustness of Euglena is supported by a substantial capacity for responding to biotic and abiotic stress: it has the capacity to deploy three separate pathways for vitamin C (ascorbate) production, as well as producing vitamin E (α-tocopherol) and, in addition to glutathione, the redox-active thiols nor-trypanothione and ovothiol.

Euglena gracilis ascorbate peroxidase forms an intramolecular dimeric structure: its unique molecular characterization

Euglena gracilis lacks a catalase and contains a single APX (ascorbate peroxidase) and enzymes related to the redox cycle of ascorbate in the cytosol. In the present study, a full-length cDNA clone encoding the Euglena APX was isolated and found to contain an open reading frame encoding a protein of 649 amino acids with a calculated molecular mass of 70.5 kDa. Interestingly, the enzyme consisted of two entirely homologous catalytic domains, designated APX-N and APX-C, and an 102 amino acid extension in the N-terminal region, which had a typical class II signal proposed for plastid targeting in Euglena. A computer-assisted analysis indicated a novel protein structure with an intramolecular dimeric structure. The analysis of cell fractionation showed that the APX protein is distributed in the cytosol, but not the plastids, suggesting that Euglena APX becomes mature in the cytosol after processing of the precursor. The kinetics of the recombinant mature FL (full-length)-APX and the APX-N and APX-C domains with ascorbate and H2O2 were almost the same as that of the native enzyme. However, the substrate specificity of the mature FL-APX and the native enzyme was different from that of APX-N and APX-C. The mature FL-APX, but not the truncated forms, could reduce alkyl hydroperoxides, suggesting that the dimeric structure is correlated with substrate recognition. In Euglena cells transfected with double-stranded RNA, the silencing of APX expression resulted in a significant increase in the cellular level of H2O2, indicating the physiological importance of APX to the metabolism of H2O2.

A high density of ancient spliceosomal introns in oxymonad excavates

Background

Certain eukaryotic genomes, such as those of the amitochondriate parasites Giardia and Trichomonas, have very low intron densities, so low that canonical spliceosomal introns have only recently been discovered through genome sequencing. These organisms were formerly thought to be ancient eukaryotes that diverged before introns originated, or at least became common. Now however, they are thought to be members of a supergroup known as excavates, whose members generally appear to have low densities of canonical introns. Here we have used environmental expressed sequence tag (EST) sequencing to identify 17 genes from the uncultivable oxymonad Streblomastix strix, to survey intron densities in this most poorly studied excavate group.

Results

We find that Streblomastix genes contain an unexpectedly high intron density of about 1.1 introns per gene. Moreover, over 50% of these are at positions shared between a broad spectrum of eukaryotes, suggesting theyare very ancient introns, potentially present in the last common ancestor of eukaryotes.

Conclusion

The Streblomastix data show that the genome of the ancestor of excavates likely contained many introns and the subsequent evolution of introns has proceeded very differently in different excavate lineages: in Streblomastix there has been much stasis while in Trichomonas and Giardia most introns have been lost.

U1 small nuclear RNA and spliceosomal introns in Euglena gracilis

In the flagellated protozoon Euglena gracilis, characterized nuclear genes harbor atypical introns that usually are flanked by short repeats, adopt complex secondary structures in pre-mRNA, and do not obey the GT-AG rule of conventional cis-spliced introns. In the nuclear fibrillarin gene of E. gracilis, we have identified three spliceosomal-type introns that have GT-AG consensus borders. Furthermore, we have isolated a small RNA from E. gracilis and propose, on the basis of primary and secondary structure comparisons, that it is a homolog of U1 small nuclear RNA, an essential component of the cis-spliceosome in higher eukaryotes. Conserved sequences at the 5' splice sites of the fibrillarin introns can potentially base pair with Euglena U1 small nuclear RNA. Our observations demonstrate that spliceosomal GT-AG cis-splicing occurs in Euglena, in addition to the nonconventional cis-splicing and spliced leader trans-splicing previously recognized in this early diverging unicellular eukaryote.

Characterization of the unique intron-exon junctions of Euglena gene(s) encoding the polyprotein precursor to the light-harvesting chlorophyll a/b binding protein of photosystem II

The precursor to the Euglena light harvesting chlorophyll a/b binding protein of photosystem II (LHCPII) is a polyprotein containing multiple copies of LHCPII covalently joined by a decapeptide linker. cDNA and genomic clones encoding the 5' and 3' end of a 6.6 kb LHCPII mRNA were sequenced. A 3.1 kb genomic region encoding 1.05 kb of the 5' end of LHCPII mRNA contains 4 introns. A 7.6 kb genomic region encoding 3.3 kb of the 3' end of LHCPII mRNA contains 10 introns. The 5' and 3' ends of the 14 identified Euglena introns lacked the conserved dinucleotides (5'-GT and AG-3') found at the termini of virtually every characterized nuclear pre-mRNA intron. A common consensus splice site selection sequence could not be identified. The Euglena introns do not have the structural characteristics of group I and group II introns. The only structural feature common to all Euglena introns was the ability of short stretches of nucleotides at the 5' and 3' ends of the introns to base pair, forming a stable stem-loop with the 5' and 3' splice site juxtaposed for splicing but displaced by 2 nucleotides. The 26 nucleotide sequence at the 5' end of LHCPII mRNA is absent from the genomic sequence and identical to the 5' end of one of the small Euglena SL-RNAs indicating that it is post-transcriptionally added by trans-splicing.