Pedal to the Metal: Nuclear Splicing Bodies Turbo-Charge VSG mRNA Production in African Trypanosomes

Abstract

The African trypanosome Trypanosoma brucei is a parasite of the mammalian bloodstream and tissues, where an antigenically variable Variant Surface Glycoprotein (VSG) coat protects it from immune attack. This dense layer comprised of ∼107 VSG proteins, makes VSG by far the most abundant mRNA (7–10% total) and protein (∼10% total) in the bloodstream form trypanosome. How can such prodigious amounts of VSG be produced from a single VSG gene? Extremely high levels of RNA polymerase I (Pol I) transcription of the active VSG provide part of the explanation. However, recent discoveries highlight the role of pre-mRNA processing, both in maintaining high levels of VSG transcription, as well as its monoallelic expression. Trypanosome mRNAs are matured through trans-splicing a spliced leader (SL) RNA to the 5’ end of precursor transcripts, meaning abundant SL RNA is required throughout the nucleus. However, requirement for SL RNA in the vicinity of the active VSG gene is so intense, that the cell reconfigures its chromatin architecture to facilitate interaction between the SL RNA genes and the active VSG. This presumably ensures that sufficient localised SL RNA is available, and not limiting for VSG mRNA expression. Recently, novel nuclear splicing bodies which appear to provide essential trans-splicing components, have been identified associating with the active VSG. These observations highlight the underappreciated role of pre-mRNA processing in modulating gene expression in trypanosomes. Dissecting the function of these nuclear RNA processing bodies should help us elucidate the mechanisms of both VSG expression and monoallelic exclusion in T. brucei.

Unwrap RAP1’s Mystery at Kinetoplastid Telomeres

“…It has been estimated that ~10 million VSG molecules are expressed on the T. brucei cell surface [193], masking invariant surface molecules from the host's immune surveillance. To make this dense VSG layer, the active VSG gene is transcribed by RNA pol I at a very high level [194], producing VSG RNA that represents ~10% of total cell RNA [283,284]. Comparing RNA levels of the same VSG gene in cells where it is either active or silent indicates that the active VSG RNA level is at least 10,000-fold higher than that of a silent VSG [103].…”

Subcellular protein localisation of Trypanosoma brucei bloodstream form-upregulated proteins maps stage-specific adaptations

“…The PCF and BSF have many well characterised differences, including the BSF VSG surface coat and associated expression machinery 3 , metabolic differences and associated remodelling of the mitochondrion 4 , morphology, and morphogenesis adaptations 5 , 6 , along with many more. However, genome-wide mapping of the global changes are broadly limited to gene expression level, most extensively determined at the mRNA level 7 – 11 which does not correlate fully with protein abundance 12 .…”

Subcellular protein localisation ofTrypanosoma bruceibloodstream form-upregulated proteins maps stage-specific adaptations

“…The PCF and BSF have many well characterised differences, including the BSF VSG surface coat and associated expression machinery [3], metabolic differences and associated remodelling of the mitochondrion [4], morphology, and morphogenesis adaptations [5,6], along with many more. However, genome-wide mapping of the global changes are broadly limited to gene expression level, most extensively determined at the mRNA level [7][8][9][10][11] which does not correlate fully with protein abundance [12].…”