Download Characterization of the trans-spliced transcriptome and polycistronic

Characterization of the trans-spliced transcriptome and polycistronic transcription units in
Echinocccus multilocularis
Ferenc Kiss1, Magdalena Zarowieck2, Uriel Koziol1, Matthew Berriman2, Klaus Brehm 1
1
Institute of Hygiene and Microbiology, University of Würzburg, Germany
2
Wellcome Trust Sanger Institute, Hinxton, Cambridge, UK
In nematodes, spliced leader (SL) trans-splicing is associated with processing of polycistronic
transcription units (PTU) into translatable mRNAs. Although SL trans-splicing is employed by all major
lineages of flatworms, extensive information on the trans-spliced transcriptome in these organisms or
on flatworm PTU was missing. Using RNA-Seq, PTU have recently been identified in the trematode
Schistosoma mansoni and herein we provide evidence that this type of gene expression is also
employed by cestodes. Based on transcriptome data, we found around 10% of E. multilocularis genes
processed involving trans-splicing. We also found that around 300 genes form part of a PTU. The
majority of Echinococcus PTU comprises two cistrons in which the downstream member is always
trans-spliced whereas the upstream partner is trans- or conventionally spliced. We also found PTU
that comprise three or four cistrons. A large number of Echinococcus PTU is also present with similar
st ructure in S. mansoni. These data indicate that many PTU were already formed early in flatworm
evolution and we indeed obtained evidence that PTU are also present in planarians. Within flatworm
PTU there is a clear prevalence of genes that encode important cell-cycle regulators, splicing factors
and mitochondrial proteins, orthologs of which are also present in nematode PTU, although transsplicing in flatworms and nematodes evolved independently. In a large number of trans-spliced genes
that are not part of a PTU we could identify 5’ UTR introns that are both conventionally spliced and
trans-spliced. This indicates that the acquisition of a 5’ UTR intron is a necessary prerequisite for
subsequent trans-splicing of a gene and eventual incorporation into a PTU. On the basis of
genome/transcriptome comparisons between cestodes, trematodes and free-living flatworms, we will
discuss the evolution of PTU in flatworms as well as a possible association of trans-splicing with gene
expression in flatworm neoblasts.
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