Abstract
A Variant Surface Glycoprotein (VSG) coat protects bloodstream form Trypanosoma brucei. Prodigious amounts of VSG mRNA (~7-10% total) are generated from a single RNA polymerase I (Pol I) transcribed VSG expression site (ES), necessitating extremely high levels of localised splicing. We show that splicing is required for processive ES transcription, and describe novel ES-associated T. brucei nuclear bodies. In bloodstream form trypanosomes, the expression site body (ESB), spliced leader array body (SLAB), NUFIP body and Cajal bodies all frequently associate with the active ES. This assembly of nuclear bodies appears to facilitate the extraordinarily high levels of transcription and splicing at the active ES. In procyclic form trypanosomes, the NUFIP body and SLAB do not appear to interact with the Pol I transcribed procyclin locus. The congregation of a restricted number of nuclear bodies at a single active ES, provides an attractive mechanism for how monoallelic ES transcription is mediated.
Highlights
A Variant Surface Glycoprotein (VSG) coat protects bloodstream form Trypanosoma brucei
In procyclic form (PF) T. brucei, the level of polymerase I (Pol I)-transcribed EP and GPEET procyclin transcripts generated per hour is 30 to 54-fold higher respectively, than from an average polymerase II (Pol II)-transcribed gene
This phenomenal rate of VSG mRNA generation is supported by extraordinarily high rates of trans-splicing at the active expression site (ES), compared with the average Pol II-transcribed gene
Summary
A Variant Surface Glycoprotein (VSG) coat protects bloodstream form Trypanosoma brucei. In bloodstream form trypanosomes, the expression site body (ESB), spliced leader array body (SLAB), NUFIP body and Cajal bodies all frequently associate with the active ES This assembly of nuclear bodies appears to facilitate the extraordinarily high levels of transcription and splicing at the active ES. VSG is produced from a single active VSG gene, yet comprises by far the most abundant mRNA and protein (~7–10% total) in BF T. brucei These extraordinarily high expression levels are facilitated by high rates of transcription initiation by Pol I, whereby subsequent transsplicing results in the generation of functional mRNA9. We investigated how these phenomenal levels of mRNA expression from a single VSG gene are achieved and explored the role of nuclear bodies including Cajal bodies in maintaining what must be extremely high levels of localised trans-splicing. The congregation of a limited number of nuclear splicing factor bodies at the active ES to form an ES nuclear body assembly provides an appealing model for how the monoallelic expression of a single ES is maintained
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