Splintrons in Giardia intestinalis

Abstract

The divergent eukaryotic unicellular organism Giardia intestinalis is an intestinal parasite in humans and various animals. An analysis of a draft genome sequence suggested that G. intestinalis has a much simpler genome organization and gene repertoire than those of other model eukaryotic organisms (e.g., Arabidopsis and human). This general picture of the G. intestinalis genome seemingly agrees with the fact that only four spliceosomal (cis-spliced) introns have been identified in this organism to date. We have recently shown that G. intestinalis possesses a unique gene expression system incorporating spliceosome-mediated trans-splicing. Some protein-coding genes in G. intestinalis are split into multiple pieces in the genome and each gene fragment is independently transcribed. Two particular pre-mRNAs directly interact with each other by forming an intermolecular-stem structure and are then trans-spliced into a mature mRNA by spliceosomes. We believe that this trans-splicing secondarily arose from the system that excises canonical (cis-splicing) introns. Based on these findings, we suspect that similar phenomena—split genes and post-transcriptional assemblage of their transcripts via trans-splicing—may be prevalent in more distinct eukaryotic lineages than previously known, particularly in organisms possessing ‘intron-poor’ genomes. 

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Acknowledgments

R.K. was a research fellow supported by JSPS for Young Scientists (210528). This work was supported by a part of a JSPS grant awarded to Y.I. (21370031) and T.H. (20570219) and a CIHR MOP-62809 grant awarded to A.J.R.

Figures and Tables

Figure 1 Depiction of intron splicing processes in Giardia. Left: Cis-splicing of a canonical spliceosomal intron. A single, continuous pre-mature mRNA (pre-mRNA) including introns is transcribed from a single locus in a genome. An arrow indicates the transcription initiation site at the 5′ upstream region of exon 1. Canonical introns are then cis-spliced by spliceosomes. Right: Trans-splicing of a splintron. Two poly-A⁺ pre-mRN As including a “left splintron piece” and a “right splintron piece” are independently transcribed from two distant loci in the genome (two different transcription initiation sites are highlighted by arrows). As the pre-mRNA forms an intermolecular stem structure, the pre-mRNA complex can be recognized as the substrate for spliceosomes.

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