scholarly journals A DOT1B/Ribonuclease H2 protein complex is involved in R-loop processing, genomic integrity and antigenic variation in Trypanosoma brucei

2020 ◽  
Author(s):  
Nicole Eisenhuth ◽  
Tim Vellmer ◽  
Falk Butter ◽  
Christian J. Janzen
Keyword(s):  
Trypanosoma Brucei ◽  
Transcriptional Activation ◽  
Antigenic Variation ◽  
Genome Integrity ◽  
Large Reservoir ◽  
Regulatory Processes ◽  
Associated Proteins ◽  
Expression Site ◽  
Protective Variant ◽  
Regulatory Functions

ABSTRACTThe parasite Trypanosoma brucei periodically changes the expression of protective variant surface glycoproteins (VSGs) to evade its host’s immune system in a process known as antigenic variation. One route to change VSG expression is the transcriptional activation of a previously silent VSG expression site (ES), a subtelomeric region containing the VSG genes. Homologous recombination of a different VSG from a large reservoir into the active ES represents another route. The conserved histone methyltransferase DOT1B is involved in transcriptional silencing of inactive ES and influences ES switching kinetics. The molecular machinery that enables DOT1B to execute these regulatory functions remains elusive, however. To better understand DOT1B-mediated regulatory processes, we purified DOT1B-associated proteins using complementary biochemical approaches. We identified several novel DOT1B-interactors. One of these was the Ribonuclease H2 complex, previously shown to resolve RNA-DNA hybrids, maintain genome integrity, and play a role in antigenic variation. Our study revealed that DOT1B depletion results in an increase in RNA-DNA hybrids, accumulation of DNA damage and recombination-based ES switching events. Surprisingly, a similar pattern of VSG deregulation was observed in Ribonuclease H2 mutants. We propose that both proteins act together in resolving R-loops to ensure genome integrity and contribute to the tightly-regulated process of antigenic variation.

scholarly journals Mapping replication dynamics in Trypanosoma brucei reveals a link with telomere transcription and antigenic variation

eLife ◽  
2016 ◽  
Vol 5 ◽  
Author(s):  
Rebecca Devlin ◽  
Catarina A Marques ◽  
Daniel Paape ◽  
Marko Prorocic ◽  
Andrea C Zurita-Leal ◽  
...  
Keyword(s):  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Replication Timing ◽  
Strand Break ◽  
Surface Glycoprotein ◽  
Genome Replication ◽  
Dna Breaks ◽  
Expression Site ◽  
Direct Generation ◽  
Protective Variant

Survival of Trypanosoma brucei depends upon switches in its protective Variant Surface Glycoprotein (VSG) coat by antigenic variation. VSG switching occurs by frequent homologous recombination, which is thought to require locus-specific initiation. Here, we show that a RecQ helicase, RECQ2, acts to repair DNA breaks, including in the telomeric site of VSG expression. Despite this, RECQ2 loss does not impair antigenic variation, but causes increased VSG switching by recombination, arguing against models for VSG switch initiation through direct generation of a DNA double strand break (DSB). Indeed, we show DSBs inefficiently direct recombination in the VSG expression site. By mapping genome replication dynamics, we reveal that the transcribed VSG expression site is the only telomeric site that is early replicating – a differential timing only seen in mammal-infective parasites. Specific association between VSG transcription and replication timing reveals a model for antigenic variation based on replication-derived DNA fragility.

scholarly journals Escaping the immune system by DNA repair and recombination in African trypanosomes

Open Biology ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 190182 ◽  
Author(s):  
Núria Sima ◽  
Emilia Jane McLaughlin ◽  
Sebastian Hutchinson ◽  
Lucy Glover
Keyword(s):  
Immune Response ◽  
Dna Repair ◽  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Surface Glycoprotein ◽  
Variant Surface Glycoprotein ◽  
Surface Coat ◽  
African Trypanosomes ◽  
Active Expression ◽  
Expression Site

African trypanosomes escape the mammalian immune response by antigenic variation—the periodic exchange of one surface coat protein, in Trypanosoma brucei the variant surface glycoprotein (VSG), for an immunologically distinct one. VSG transcription is monoallelic, with only one VSG being expressed at a time from a specialized locus, known as an expression site. VSG switching is a predominantly recombination-driven process that allows VSG sequences to be recombined into the active expression site either replacing the currently active VSG or generating a ‘new’ VSG by segmental gene conversion. In this review, we describe what is known about the factors that influence this process, focusing specifically on DNA repair and recombination.

scholarly journals Trypanosoma brucei PolIE suppresses telomere recombination

2021 ◽  
Author(s):  
Maiko Tonini ◽  
M. A. G. Rabbani ◽  
Marjia Afrin ◽  
Bibo Li
Keyword(s):  
Trypanosoma Brucei ◽  
Immune Evasion ◽  
Human African Trypanosomiasis ◽  
Antigenic Variation ◽  
Telomere Maintenance ◽  
Genome Integrity ◽  
Major Player ◽  
Elevated Rate ◽  
Major Surface ◽  
Telomere Structure

Telomeres are essential for genome integrity and stability. In T. brucei that causes human African trypanosomiasis, the telomere structure and telomere proteins also influence the virulence of the parasite, as its major surface antigen involved in the host immune evasion is expressed exclusively from loci immediately upstream of the telomere repeats. However, telomere maintenance mechanisms are still unclear except that telomerase-mediated telomere synthesis is a major player. We now identify PolIE as an intrinsic telomere complex component. We find that depletion of PolIE leads to an increased amount of telomere/subtelomere DNA damage, an elevated rate of antigenic variation, and an increased amount of telomere T-circles and C-circles, indicating that PolIE suppresses telomere recombination and helps maintain telomere integrity. In addition, we observe much longer telomere G-rich 3 prime overhangs in PolIE-depleted cells, which is not dependent on telomerase. Furthermore, the level of telomere DNA synthesis is slightly increased in PolIE-depleted cells, which is dependent on telomerase. Therefore, we identify PolIE as a major player for telomere maintenance in T. brucei.

scholarly journals Expression site attenuation mechanistically links antigenic variation and development in Trypanosoma brucei

eLife ◽  
2014 ◽  
Vol 3 ◽  
Author(s):  
Christopher Batram ◽  
Nicola G Jones ◽  
Christian J Janzen ◽  
Sebastian M Markert ◽  
Markus Engstler
Keyword(s):  
Trypanosoma Brucei ◽  
Histone Methylation ◽  
Antigenic Variation ◽  
Tsetse Fly ◽  
Developmental Competence ◽  
Surface Glycoprotein ◽  
G1 Phase ◽  
African Trypanosomes ◽  
Developmental Progression ◽  
Expression Site

We have discovered a new mechanism of monoallelic gene expression that links antigenic variation, cell cycle, and development in the model parasite Trypanosoma brucei. African trypanosomes possess hundreds of variant surface glycoprotein (VSG) genes, but only one is expressed from a telomeric expression site (ES) at any given time. We found that the expression of a second VSG alone is sufficient to silence the active VSG gene and directionally attenuate the ES by disruptor of telomeric silencing-1B (DOT1B)-mediated histone methylation. Three conserved expression-site-associated genes (ESAGs) appear to serve as signal for ES attenuation. Their depletion causes G1-phase dormancy and reversible initiation of the slender-to-stumpy differentiation pathway. ES-attenuated slender bloodstream trypanosomes gain full developmental competence for transformation to the tsetse fly stage. This surprising connection between antigenic variation and developmental progression provides an unexpected point of attack against the deadly sleeping sickness.

scholarly journals Author response: Expression site attenuation mechanistically links antigenic variation and development in Trypanosoma brucei

2014 ◽  
Author(s):  
Christopher Batram ◽  
Nicola G Jones ◽  
Christian J Janzen ◽  
Sebastian M Markert ◽  
Markus Engstler
Keyword(s):  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Author Response ◽  
Expression Site ◽  
Response Expression

scholarly journals Quantification of RNA Polymerase I transcriptional attenuation at the active VSG expression site in Trypanosoma brucei

2021 ◽  
Author(s):  
Nadine Weisert ◽  
Klara Thein ◽  
Helena Reis ◽  
Christian J Janzen
Keyword(s):  
Rna Polymerase ◽  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Rna Polymerase I ◽  
Surface Glycoprotein ◽  
Transcriptional Elongation ◽  
Pol I ◽  
Expression Site ◽  
Polymerase I ◽  
Large Repertoire

The cell surface of the extracellular pathogen Trypanosoma brucei consists of a dense coat of variant surface glycoprotein (VSG), which enables the parasite to evade the immune system of the vertebrate host. Only one VSG gene from a large repertoire is expressed from a so-called bloodstream form expression site (BES) at a given timepoint. There are several BES in every parasite but only one is transcriptionally active. Other BES are silenced by transcriptional attenuation. Periodic activation of a previously-silenced BES results in differential VSG transcription and escape from the immune response. A process called antigenic variation. In contrast to gene transcription in other eukaryotes, the BES is transcribed by RNA polymerase I (Pol I). It was proposed that this highly-processive polymerase is needed to provide a sufficiently high transcription rate at the VSG gene. Surprisingly, we discovered a position-dependent Pol I activity and attenuation of transcriptional elongation also at the active BES. Transcription rates at the VSG gene appear to be comparable to Pol II-mediated transcription of house-keeping genes. Although these findings are in contradiction to the long-standing concept of continuously high transcription rates at the active BES in Trypanosoma brucei, they are complementary to recent groundbreaking findings about transcriptional regulation of VSG genes.

scholarly journals Nuclear repositioning of the VSG promoter during developmental silencing in Trypanosoma brucei

2007 ◽  
Vol 176 (2) ◽  
pp. 133-139 ◽  
Author(s):  
David Landeira ◽  
Miguel Navarro
Keyword(s):  
Trypanosoma Brucei ◽  
Transcriptional Activation ◽  
Promoter Region ◽  
Chromatin Condensation ◽  
Surface Protein ◽  
Model System ◽  
Surface Glycoprotein ◽  
Pol I ◽  
Expression Site ◽  
Developmental Differentiation

Interphase nuclear repositioning of chromosomes has been implicated in the epigenetic regulation of RNA polymerase (pol) II transcription. However, little is known about the nuclear position–dependent regulation of RNA pol I–transcribed loci. Trypanosoma brucei is an excellent model system to address this question because its two main surface protein genes, procyclin and variant surface glycoprotein (VSG), are transcribed by pol I and undergo distinct transcriptional activation or downregulation events during developmental differentiation. Although the monoallelically expressed VSG locus is exclusively localized to an extranucleolar body in the bloodstream form, in this study, we report that nonmutually exclusive procyclin genes are located at the nucleolar periphery. Interestingly, ribosomal DNA loci and pol I transcription activity are restricted to similar perinucleolar positions. Upon developmental transcriptional downregulation, however, the active VSG promoter selectively undergoes a rapid and dramatic repositioning to the nuclear envelope. Subsequently, the VSG promoter region was subjected to chromatin condensation. We propose a model whereby the VSG expression site pol I promoter is selectively targeted by temporal nuclear repositioning during developmental silencing.

scholarly journals Keeping Balance Between Genetic Stability and Plasticity at the Telomere and Subtelomere of Trypanosoma brucei

2021 ◽  
Vol 9 ◽  
Author(s):  
Bibo Li
Keyword(s):  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Microbial Pathogens ◽  
Yeast Cells ◽  
Genome Integrity ◽  
Environmental Cues ◽  
Telomere Protein ◽  
Nucleoprotein Complexes ◽  
Underlying Mechanisms ◽  
Kinetoplastid Parasite

Telomeres, the nucleoprotein complexes at chromosome ends, are well-known for their essential roles in genome integrity and chromosome stability. Yet, telomeres and subtelomeres are frequently less stable than chromosome internal regions. Many subtelomeric genes are important for responding to environmental cues, and subtelomeric instability can facilitate organismal adaptation to extracellular changes, which is a common theme in a number of microbial pathogens. In this review, I will focus on the delicate and important balance between stability and plasticity at telomeres and subtelomeres of a kinetoplastid parasite, Trypanosoma brucei, which causes human African trypanosomiasis and undergoes antigenic variation to evade the host immune response. I will summarize the current understanding about T. brucei telomere protein complex, the telomeric transcript, and telomeric R-loops, focusing on their roles in maintaining telomere and subtelomere stability and integrity. The similarities and differences in functions and underlying mechanisms of T. brucei telomere factors will be compared with those in human and yeast cells.

Sign in / Sign up

Export Citation Format

Share Document