Histone H3 Variant Regulates RNA Polymerase II Transcription Termination and Dual Strand Transcription of siRNA Loci in Trypanosoma brucei

Reynolds, David M.; Hofmeister, Brigitte T.; Cliffe, Laura J.; Alabady, Magdy S.; Siegel, T. Nicolai; Schmitz, Robert J.; Sabatini, Robert

doi:10.1371/journal.pgen.1005758

Cited by 57 publications

(116 citation statements)

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“…For example, a histone H3 variant, a bloodstream stagespecific modified DNA base known as J or hydroxymethyluracil (22,23), the chromatin remodeling ISWI complex (24), the histone H3K76 trimethyltransferase DOT1B (25), and the telomere-associated protein RAP1 (26) all facilitate VSG-ES silencing. In addition, cohesin function facilitates maintenance of the active VSG-ES (27), and inositol phosphate signaling impacts VSG-ES regulation (28).…”

mentioning

confidence: 99%

VEX1 controls the allelic exclusion required for antigenic variation in trypanosomes

Glover

Hutchinson

Alsford

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

184

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Allelic exclusion underpins antigenic variation and immune evasion in African trypanosomes. These bloodstream parasites use RNA polymerase-I (pol-I) to transcribe just one telomeric variant surface glycoprotein (VSG) gene at a time, producing superabundant and switchable VSG coats. We identified trypanosome VSG exclusion-1 (VEX1) using a genetic screen for defects in telomere-exclusive expression. VEX1 was sequestered by the active VSG and silencing of other VSGs failed when VEX1 was either ectopically expressed or depleted, indicating positive and negative regulation, respectively. Positive regulation affected VSGs and nontelomeric pol-I-transcribed genes, whereas negative regulation primarily affected VSGs. Negative regulation by VEX1 also affected telomeric pol-I-transcribed reporter constructs, but only when they contained blocks of sequence sharing homology with a pol-I-transcribed locus. We conclude that restricted positive regulation due to VEX1 sequestration, combined with VEX1-dependent, possibly homology-dependent silencing, drives a "winner-takes-all" mechanism of allelic exclusion.epigenetic | monoallelic | silencing | telomere | Trypanosoma brucei C ells often restrict expression to a single allele of a gene or gene family. This allelic exclusion underpins antigenic variation in pathogens, including trypanosomes that cause sleeping sickness (1) and Plasmodium parasites that cause malaria (2). Allelic exclusion is also essential for singular olfactory receptor expression and a sense of smell in metazoa (3). Although many factors have been identified that are required for the expression of one allele or for the silencing of other alleles in these systems, our understanding of the mechanisms by which expression and silencing are established and coordinated remains incomplete (1-3).The African trypanosome Trypanosoma brucei is a flagellated parasitic protozoan transmitted among mammalian hosts by tsetse flies. In addition to causing trypanosomiasis in humans, a fatal and neglected tropical disease, these parasites also cause nagana in cattle. Antigenic variation is essential for persistent bloodstream infection in the face of host adaptive immune defenses and has long been a paradigm for studies on allelic exclusion (1); parasite immune evasion depends upon singular variant surface glycoprotein (VSG) gene expression and VSG switching. Although multiple subtelomeric VSGs are available for expression (4), only one is transcribed (5). Both active and silent VSGs are located at the ends of polycistronic transcription units known as expression sites (ESs) (6). Notably, VSG-ES promoters (6) recruit RNA polymerase-I (pol-I) that typically transcribes ribosomal RNA genes (7). Indeed, the active VSG-ES is associated with an extranucleolar focus of pol-I known as the expression-site body (ESB) (8-10). Although the active VSG-ES is specifically depleted of nucleosomes (11, 12), silent VSG-ESs are similarly located in the extranucleolar space in bloodstream-form cells, and neither active nor silent VSG-ESs show an ...

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confidence: 99%

VEX1 controls the allelic exclusion required for antigenic variation in trypanosomes

Glover

Hutchinson

Alsford

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

184

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“…Finally, although it has been long thought that RNA Pol II transcriptional regulation does not operate in these organisms, recent reports have suggested that trypanosomes could regulate RNA Pol II transcription by epigenetic mechanisms (Ekanayake and Sabatini ; Reynolds et al. ; Stanne and Rudenko ). Results reported in this paper present the first experimental evidence showing that the RBP TbRRM1 plays a key role in RNA Pol II transcriptional regulation in T. brucei .…”

Section: Discussionmentioning

confidence: 99%

The Trypanosoma brucei RNA‐Binding Protein TbRRM1 is Involved in the Transcription of a Subset of RNA Pol II‐Dependent Genes

Bañuelos

Levy

Níttolo

et al. 2019

J Eukaryotic Microbiology

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It has been long thought that RNA Polymerase (Pol) II transcriptional regulation does not operate in trypanosomes. However, recent reports have suggested that these organisms could regulate RNA Pol II transcription by epigenetic mechanisms. In this paper, we investigated the role of TbRRM1 in transcriptional regulation of RNA Pol II‐dependent genes by focusing both in genes located in a particular polycistronic transcription unit (PTU) and in the monocistronic units of the SL‐RNA genes. We showed that TbRRM1 is recruited throughout the PTU, with a higher presence on genes than intergenic regions. However, its depletion leads both to the decrease of nascent RNA and to chromatin compaction only of regions located distal to the main transcription start site. These findings suggest that TbRRM1 facilitates the RNA Pol II transcriptional elongation step by collaborating to maintain an open chromatin state in particular regions of the genome. Interestingly, the SL‐RNA genes do not recruit TbRRM1 and, after TbRRM1 knockdown, nascent SL‐RNAs accumulate while the chromatin state of these regions remains unchanged. Although it was previously suggested that TbRRM1 could regulate RNA Pol II‐driven genes, we provide here the first experimental evidence which involves TbRRM1 to transcriptional regulation.

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“…In contrast, silent ESs are marked with an unusual modified glucosylated nucleotide called Base J, which is incorporated in a gradient that increases towards the telomere end and could play some role in silencing these telomeric VSGs [34,35]. Additionally, different histone variants and modifications have also been shown to play a role in the down-regulation of ESs (Figure 2).…”

Section: How Is Vsg Expression Controlled In a Monoallelic Fashion?mentioning

confidence: 99%

“…The histone variant H3.V is enriched around the termination region of Pol II transcription units [25]. Knockdown of histone variant H3.V not only results in the disruption of Pol II transcription termination but also causes the derepression of the silent telomeric ESs [34,35]. Histone modifications also play an important role in ES control.…”

Section: How Is Vsg Expression Controlled In a Monoallelic Fashion?mentioning

confidence: 99%

How to create coats for all seasons: elucidating antigenic variation in African trypanosomes

Ooi

Rudenko

2017

Emerging Topics in Life Sciences

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Extracellular parasites of the mammalian bloodstream face considerable challenges including incessant assault by the immune system. African trypanosomes are consummate survivors in this inclement environment and are renowned for their supremely sophisticated strategy of antigenic variation of their protective surface coat during the course of chronic infections. Recent developments are making us realize how complex this antigenic machinery is and are allowing us to tackle previously intractable problems. However, many of the simplest (and arguably the most important) questions still remain unanswered! How is the extraordinary heterogeneity of variant surface glycoprotein variants during a chronic infection generated?The bloodstream form of Trypanosoma brucei is coated with a dense covering of ∼10 7 variant surface glycoprotein (VSG) molecules, which extend as antigenically variable rods connected to the cell surface via glycosylphosphatidylinositol (GPI) anchors [1]. This GPI attachment allows VSGs to move freely over the cell surface [2]. Tight packing of these VSGs produces a protective barrier shielding invariant surface proteins from recognition by host antibodies and preventing trypanosome lysis by the alternative pathway of the complement system [3]. Extraordinarily high rates of recycling of surface VSG allow selective removal of host cell molecules including antibodies and complement from the trypanosome surface, thereby functioning as a 'coat-cleaning ' machine [4]. This prolongs trypanosome survival in rising antibody titres and facilitates escape from macrophages [5].Each trypanosome has a vast repertoire of VSG genes [6], differing in number and content between different strains, presumably as a consequence of diversifying forces. For example, the 'VSGnome' of T. brucei 427 contains more than 2500 different VSG genes, of which more than 80% are not immediately functional [7]. Switching the active VSG can entail transcriptional switches between the ∼15 telomeric VSG expression site (ES) transcription units (Figure 1). More importantly, DNA rearrangements and predominantly gene conversions can copy new VSGs (or segments of VSGs) into the active ES, thereby resulting in an antigenic switch [8] (Figure 1). This extraordinary ability to construct novel patchwork, or mosaic, VSGs is key for trypanosome survival [9]. The continuous recombination of various permutations and combinations of sections of VSG genes (or pseudogenes) allows the trypanosome to construct an almost infinite number of antigenically distinct VSG coat types.A given wave of trypanosome parasitaemia is normally predominantly composed of a major VSG variant, although it has long been known that minor VSG variants are also present within each wave. However, there is possibly phenomenal heterogeneity within these infection waves, as next-generation RNA sequencing of RNA transcripts has documented an extraordinary variety of minor VSG variants [10]. These experiments argue that a considerable percentage of the available VSG repertoire is disp...

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Histone H3 Variant Regulates RNA Polymerase II Transcription Termination and Dual Strand Transcription of siRNA Loci in Trypanosoma brucei

Cited by 57 publications

References 96 publications

VEX1 controls the allelic exclusion required for antigenic variation in trypanosomes

VEX1 controls the allelic exclusion required for antigenic variation in trypanosomes

The Trypanosoma brucei RNA‐Binding Protein TbRRM1 is Involved in the Transcription of a Subset of RNA Pol II‐Dependent Genes

How to create coats for all seasons: elucidating antigenic variation in African trypanosomes

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