A monocistronic transcript for a trypanosome variant surface glycoprotein

VEX1 Influences mVSG Expression During the Transition to Mammalian Infectivity in Trypanosoma brucei

The Trypanosoma (T) brucei life cycle alternates between the tsetse fly vector and the mammalian host. In the insect, T. brucei undergoes several developmental stages until it reaches the salivary gland and differentiates into the metacyclic form, which is capable of infecting the next mammalian host. Mammalian infectivity is dependent on expression of the metacyclic variant surface glycoprotein genes as the cells develop into mature metacyclics. The VEX complex is essential for monoallelic variant surface glycoprotein expression in T. brucei bloodstream form, however, initiation of expression of the surface proteins genes during metacyclic differentiation is poorly understood. To better understand the transition to mature metacyclics and the control of metacyclic variant surface glycoprotein expression we examined the role of VEX1 in this process. We show that modulating VEX1 expression leads to a dysregulation of variant surface glycoprotein expression during metacyclogenesis, and that following both in vivo and in vitro metacyclic differentiation VEX1 relocalises from multiple nuclear foci in procyclic cells to one to two distinct nuclear foci in metacyclic cells - a pattern like the one seen in mammalian infective bloodstream forms. Our data suggest a role for VEX1 in the metacyclic differentiation process and their capacity to become infectious to the mammalian host.

Temperature shift activates bloodstream VSG expression site promoters in Trypanosoma brucei

Trypanosoma brucei relies on two types of variant surface glycoprotein (VSG) expression sites (ESs) for RNA polymerase I (Pol I) transcription of VSG pre-mRNA. Trypanosomes developing into infectious metacyclic cells in the tsetse vector use metacyclic VSG ESs (MESs) and proliferating parasites in the mammalian host deploy bloodstream VSG ESs (BESs). Unlike the monocistronic MESs, BESs are polycistronic and their highly conserved promoters differ considerably from the MES promoters. The significance of the divergent sequences of MES and BES promoters remains to be determined. We used a reporter system to specifically test the effect of temperature on the activity of MES and BES promoters in procyclic trypanosomes and our results demonstrate that transcription from the MES promoter is largely insensitive to changes in temperature. In contrast, the BES promoter drives rapid activation of transcription upon a change of temperature from 28 °C to 37 °C. Additionally, endogenous BESs respond similarly to the elevation of temperature and initiate increased production of BES pre-mRNA and mRNA. Our results indicate that the sequence of the BES promoter is a specificity signal which triggers BES activation in vivo upon entry into the mammalian host.

A single-point mutation in the RNA-binding protein 6 generates Trypanosoma brucei metacyclics that are able to progress to bloodstream forms in vitro

We previously established an in vitro differentiation system based on the inducible expression of the RNA binding protein 6 (RBP6), which initiated differentiation of Trypanosoma brucei non-infectious procyclics to infectious metacyclics (MFs). However, further differentiation to bloodstream forms (BFs) required infection of mice. Here we report the serendipitous isolation of a single point mutation in RBP6 (Q109K), whose expression not only generated MFs, but purified MFs continued the developmental cycle in vitro to BFs expressing variant surface glycoprotein-2 (VSG-2), formerly known as VSG 221. This transition occurred over a period of 11 days and by RNA-Seq, VSG-2 was first measureable on day 1, whereas metacyclic VSGs were detected up to 8 days. We further showed that inducible expression of mutant RBP6 appeared to skip the intermediate epimastigote stage and we highlight the potential involvement of RBP33 in the establishment of metacyclics and in particular in the generation of metacyclics uncharacteristically arrested at the G2/M checkpoint.

The proteome and transcriptome of the infectious metacyclic form of Trypanosoma brucei define quiescent cells primed for mammalian invasion

The infectious metacyclic forms of Trypanosoma brucei result from a complex development in the tsetse fly vector. When they infect mammals, they cause African sleeping sickness in humans. Due to scarcity of biological material and difficulties of the tsetse fly as an experimental system, very limited information is available concerning the gene expression profile of metacyclic forms. We used an in vitro system based on expressing the RNA binding protein 6 to obtain infectious metacyclics and determined their protein and mRNA repertoires by mass-spectrometry (MS) based proteomics and mRNA sequencing (RNA-Seq) in comparison to non-infectious procyclic trypanosomes. We showed that metacyclics are quiescent cells, and propose this influences the choice of a monocistronic variant surface glycoprotein expression site. Metacyclics have a largely bloodstream-form type transcriptome, and thus are programmed to translate a bloodstream-form type proteome upon entry into the mammalian host and resumption of cell division. Genes encoding cell surface components showed the largest changes between procyclics and metacyclics, observed at both the transcript and protein levels. Genes encoding metabolic enzymes exhibited expression in metacyclics with features of both procyclic and bloodstream forms, suggesting that this intermediate-type metabolism is dictated by the availability of nutrients in the tsetse fly vector.

Metacyclic VSG expression site promoters are recognized by the same general transcription factor that is required for RNA polymerase I transcription of bloodstream expression sites

Infectious metacyclic Trypanosoma brucei cells develop in the salivary glands of tsetse flies. A critical aspect of the developmental program leading to acquisition of infectivity is the synthesis of a variant surface glycoprotein (VSG) coat. Metacyclic VSG genes are transcribed from a set of specialized VSG expression sites (ESs) that differ from bloodstream VSG ESs by being monocistronic, being significantly shorter, lacking long stretches of 70-bp repeats, and having distinct promoter sequences. Both metacyclic and bloodstream VSG ESs are transcribed by the multifunctional T. brucei RNA polymerase I (Pol I), however the factor that recognizes the divergent metacyclic VSG ES promoters and recruits Pol I during the development to infectious cells remains unknown. We used an in vitro assay to show that the promoters for both metacyclic and bloodstream VSG ESs are recognized by the same class I transcription factor A (CITFA). This general Pol I transcription initiation factor was previously shown to be essential for the transcription of bloodstream VSG genes, procyclin genes and rRNA genes, and was demonstrated to have distinct binding affinities for these three types of promoters. We now show that differences in the sequence of individual metacyclic VSG ESs promoters determine different affinities for CITFA.

Transcriptome Profiling of Trypanosoma brucei Development in the Tsetse Fly Vector Glossina morsitans

African trypanosomes, the causative agents of sleeping sickness in humans and nagana in animals, have a complex digenetic life cycle between a mammalian host and an insect vector, the blood-feeding tsetse fly. Although the importance of the insect vector to transmit the disease was first realized over a century ago, many aspects of trypanosome development in tsetse have not progressed beyond a morphological analysis, mainly due to considerable challenges to obtain sufficient material for molecular studies. Here, we used high-throughput RNA-Sequencing (RNA-Seq) to profile Trypanosoma brucei transcript levels in three distinct tissues of the tsetse fly, namely the midgut, proventriculus and salivary glands. Consistent with current knowledge and providing a proof of principle, transcripts coding for procyclin isoforms and several components of the cytochrome oxidase complex were highly up-regulated in the midgut transcriptome, whereas transcripts encoding metacyclic VSGs (mVSGs) and the surface coat protein brucei alanine rich protein or BARP were extremely up-regulated in the salivary gland transcriptome. Gene ontology analysis also supported the up-regulation of biological processes such as DNA metabolism and DNA replication in the proventriculus transcriptome and major changes in signal transduction and cyclic nucleotide metabolism in the salivary gland transcriptome. Our data highlight a small repertoire of expressed mVSGs and potential signaling pathways involving receptor-type adenylate cyclases and members of a surface carboxylate transporter family, called PADs (Proteins Associated with Differentiation), to cope with the changing environment, as well as RNA-binding proteins as a possible global regulators of gene expression.

The nuclear envelope and gene organization in parasitic protozoa: Specializations associated with disease

The parasitic protozoa Trypanosoma brucei and Plasmodium falciparum are lethal human parasites that have developed elegant strategies of immune evasion by antigenic variation. Despite the vast evolutionary distance between the two taxa, both parasites employ strict monoallelic expression of their membrane proteins, variant surface glycoproteins in Trypanosomes and the var, rif and stevor genes in Plasmodium, in order to evade their host's immune system. Additionally, both telomeric location and epigenetic controls are prominent features of these membrane proteins. As such, telomeres, chromatin structure and nuclear organization all contribute to control of gene expression and immune evasion. Here, we discuss the importance of epigenetics and sub-nuclear context for the survival of these disease-causing parasites.

Synchronous expression of individual metacyclic variant surface glycoprotein genes in Trypanosoma brucei

One distinctive feature of the Trypanosoma brucei life cycle is the presence of two discrete populations that are based on differential expression of variant surface glycoproteins (VSGs). Both are adapted to the environmental pressures they face and more importantly, both contribute directly to transmission. Metacyclics in the tsetse fly enable transmission to a new mammalian host, whereas bloodstream trypanosomes must avoid immune destruction to the extent that sufficient numbers are available for transmission, when the insect vector takes a blood meal. At present, there are few investigations on the molecular aspects of parasite biology in the tsetse vector and specifically about the activation of metacyclic VSG gene expression. Here we used an established in vitro differentiation system based on the overexpression of the RNA-binding protein 6 (RBP6), to monitor two metacyclic VSGs (VSG 397 and VSG 653) during development from procyclics to infectious metacyclic forms. We observed that activation of these two mVSGs was simultaneous both at the transcript and protein level, and manifested by the appearance of only one of the mVSGs in individual cells.

A Host-Pathogen Interaction Reduced to First Principles: Antigenic Variation in T. brucei

Antigenic variation is a common microbial survival strategy, powered by diversity in expressed surface antigens across the pathogen population over the course of infection. Even so, among pathogens, African trypanosomes have the most comprehensive system of antigenic variation described. African trypanosomes (Trypanosoma brucei spp.) are unicellular parasites native to sub-Saharan Africa, and the causative agents of sleeping sickness in humans and of n'agana in livestock. They cycle between two habitats: a specific species of fly (Glossina spp. or, colloquially, the tsetse) and the bloodstream of their mammalian hosts, by assuming a succession of proliferative and quiescent developmental forms, which vary widely in cell architecture and function. Key to each of the developmental forms that arise during these transitions is the composition of the surface coat that covers the plasma membrane. The trypanosome surface coat is extremely dense, covered by millions of repeats of developmentally specified proteins: procyclin gene products cover the organism while it resides in the tsetse and metacyclic gene products cover it while in the fly salivary glands, ready to make the transition to the mammalian bloodstream. But by far the most interesting coat is the Variant Surface Glycoprotein (VSG) coat that covers the organism in its infectious form (during which it must survive free living in the mammalian bloodstream). This coat is highly antigenic and elicits robust VSG-specific antibodies that mediate efficient opsonization and complement mediated lysis of the parasites carrying the coat against which the response was made. Meanwhile, a small proportion of the parasite population switches coats, which stimulates a new antibody response to the prevalent (new) VSG species and this process repeats until immune system failure. The disease is fatal unless treated, and treatment at the later stages is extremely toxic. Because the organism is free living in the blood, the VSG:antibody surface represents the interface between pathogen and host, and defines the interaction of the parasite with the immune response. This interaction (cycles of VSG switching, antibody generation, and parasite deletion) results in stereotypical peaks and troughs of parasitemia that were first recognized more than 100 years ago. Essentially, the mechanism of antigenic variation in T. brucei results from a need, at the population level, to maintain an extensive repertoire, to evade the antibody response. In this chapter, we will examine what is currently known about the VSG repertoire, its depth, and the mechanisms that diversify it both at the molecular (DNA) and at the phenotypic (surface displayed) level, as well as how it could interact with antibodies raised specifically against it in the host.

Capturing the variant surface glycoprotein repertoire (the VSGnome) of Trypanosoma brucei Lister 427

Trypanosoma brucei evades the adaptive immune response through the expression of antigenically distinct Variant Surface Glycoprotein (VSG) coats. To understand the progression and mechanisms of VSG switching, and to identify the VSGs expressed in populations of trypanosomes, it is desirable to predetermine the available repertoire of VSG genes (the 'VSGnome'). To date, the catalog of VSG genes present in any strain is far from complete and the majority of current information regarding VSGs is derived from the TREU927 strain that is not commonly used as an experimental model. We have assembled, annotated and analyzed 2563 distinct and previously unsequenced genes encoding complete and partial VSGs of the widely used Lister 427 strain of T. brucei. Around 80% of the VSGnome consists of incomplete genes or pseudogenes. Read-depth analysis demonstrated that most VSGs exist as single copies, but 360 exist as two or more indistinguishable copies. The assembled regions include five functional metacyclic VSG expression sites. One third of minichromosome sub-telomeres contain a VSG (64-67 VSGs on ∼96 minichromosomes), of which 85% appear to be functionally competent. The minichromosomal repertoire is very dynamic, differing among clones of the same strain. Few VSGs are unique along their entire length: frequent recombination events are likely to have shaped (and to continue to shape) the repertoire. In spite of their low sequence conservation and short window of expression, VSGs show evidence of purifying selection, with ∼40% of non-synonymous mutations being removed from the population. VSGs show a strong codon-usage bias that is distinct from that of any other group of trypanosome genes. VSG sequences are generally very divergent between Lister 427 and TREU927 strains of T. brucei, but those that are highly similar are not found in 'protected' genomic environments, but may reflect genetic exchange among populations.

Antigenic variation in African trypanosomes

Studies on Variant Surface Glycoproteins (VSGs) and antigenic variation in the African trypanosome, Trypanosoma brucei, have yielded a remarkable range of novel and important insights. The features first identified in T. brucei extend from unique to conserved-among-trypanosomatids to conserved-among-eukaryotes. Consequently, much of what we now know about trypanosomatid biology and much of the technology available has its origin in studies related to VSGs. T. brucei is now probably the most advanced early branched eukaryote in terms of experimental tractability and can be approached as a pathogen, as a model for studies on fundamental processes, as a model for studies on eukaryotic evolution or often all of the above. In terms of antigenic variation itself, substantial progress has been made in understanding the expression and switching of the VSG coat, while outstanding questions continue to stimulate innovative new approaches. There are large numbers of VSG genes in the genome but only one is expressed at a time, always immediately adjacent to a telomere. DNA repair processes allow a new VSG to be copied into the single transcribed locus. A coordinated transcriptional switch can also allow a new VSG gene to be activated without any detectable change in the DNA sequence, thereby maintaining singular expression, also known as allelic exclusion. I review the story behind VSGs; the genes, their expression and switching, their central role in T. brucei virulence, the discoveries that emerged along the way and the persistent questions relating to allelic exclusion in particular.

Silencing subtelomeric VSGs by Trypanosoma brucei RAP1 at the insect stage involves chromatin structure changes

Trypanosoma brucei causes human African trypanosomiasis and regularly switches its major surface antigen variant surface glycoprotein (VSG) to evade mammalian host immune responses at the bloodstream form (BF) stage. Monoallelic expression of BF Expression Site (BES)-linked VSGs and silencing of metacyclic VSGs (mVSGs) in BF cells are essential for antigenic variation, whereas silencing of both BES-linked and mVSGs in the procyclic form (PF) cells is important for cell survival in the midgut of its insect vector. We have previously shown that silencing BES-linked VSGs in BF cells depends on TbRAP1. We now show that TbRAP1 silences both BES-linked and mVSGs at both BF and PF stages. The strength of TbRAP1-mediated BES-linked VSG silencing is stronger in the PF cells than that in BF cells. In addition, Formaldehyde-Assisted Isolation of Regulatory Elements analysis and MNase digestion demonstrated that depletion of TbRAP1 in PF cells led to a chromatin structure change, which is significantly stronger at the subtelomeric VSG loci than at chromosome internal loci. On the contrary, no significant chromatin structure changes were detected on depletion of TbRAP1 in BF cells. Our observations indicate that TbRAP1 helps to determine the chromatin structure at the insect stage, which likely contributes to its strong silencing effect on VSGs.

MCM-BP is required for repression of life-cycle specific genes transcribed by RNA polymerase I in the mammalian infectious form of Trypanosoma brucei

Trypanosoma brucei variant surface glycoprotein (VSG) expression is a classic example of allelic exclusion. While the genome of T. brucei contains >2,000 VSG genes and VSG pseudogenes, only one allele is expressed at the surface of each infectious trypanosome and the others are repressed. Along with recombinatorial VSG switching, allelic exclusion provides a major host evasion mechanism for trypanosomes, a phenomenon known as antigenic variation. To extend our understanding of how trypanosomes escape host immunity by differential expression of VSGs, we attempted to identify genes that contribute to VSG silencing, by performing a loss-of-silencing screen in T. brucei using a transposon-mediated random insertional mutagenesis. One identified gene, which we initially named LOS1, encodes a T. brucei MCM-Binding Protein (TbMCM-BP). Here we show that TbMCM-BP is essential for viability of infectious bloodstream-form (BF) trypanosome and is required for proper cell-cycle progression. Tandem affinity purification of TbMCM-BP followed by mass spectrometry identified four subunits (MCM4-MCM7) of the T. brucei MCM complex, a replicative helicase, and MCM8, a subunit that is uniquely co-purified with TbMCM-BP. TbMCM-BP is required not only for repression of subtelomeric VSGs but also for silencing of life-cycle specific, insect-stage genes, procyclin and procyclin-associated genes (PAGs), that are normally repressed in BF trypanosomes and are transcribed by RNA polymerase I. Our study uncovers a functional link between chromosome maintenance and RNA pol I-mediated gene silencing in T. brucei.

Cytokinesis of Trypanosoma brucei bloodstream forms depends on expression of adenylyl cyclases of the ESAG4 or ESAG4-like subfamily

Antigenic variation of the parasite Trypanosoma brucei operates by monoallelic expression of a variant surface glycoprotein (VSG) from a collection of multiple telomeric expression sites (ESs). Each of these ESs harbours a long polycistronic transcription unit containing several expression site-associated genes (ESAGs). ESAG4 copies encode bloodstream stage-specific adenylyl cyclases (AC) and belong to a larger gene family of around 80 members, the majority of which, termed genes related to ESAG4 (GRESAG4s), are not encoded in ESs and are expressed constitutively in the life cycle. Here we report that ablation of ESAG4 from the active ES did not affect parasite growth, neither in culture nor upon rodent infection, and did not significantly change total AC activity. In contrast, inducible RNAi-mediated knock-down of an AC subfamily that includes ESAG4 and two ESAG4-like GRESAG4 (ESAG4L) genes, decreased total AC activity and induced a lethal phenotype linked to impaired cytokinesis. In the Δesag4 line compensatory upregulation of apparently functionally redundant ESAG4L genes was observed, suggesting that the ESAG4/ESAG4L-subfamily ACs are involved in the control of cell division. How deregulated adenylyl cyclases or cAMP might impair cytokinesis is discussed.

Trypanosoma brucei: a putative RNA polymerase II promoter

RNA polymerase II (pol II) promoters are rare in the African trypanosome Trypanosoma brucei because gene regulation in the parasite is complex and polycistronic. Here, we describe a putative pol II promoter and its structure-function relationship. The promoter has features of an archetypal eukaryotic pol II promoter including putative canonical CCAAT and TATA boxes, and an initiator element. However, the spatial arrangement of these elements is only similar to yeast pol II promoters. Deletion mapping and transcription assays enabled delineation of a minimal promoter that could drive orientation-independent reporter gene expression suggesting that it may be a bidirectional promoter. In vitro transcription in a heterologous nuclear extract revealed that the promoter can be recognized by the basal eukaryotic transcription complex. This suggests that the transcription machinery in the parasite may be very similar to those of other eukaryotes.

Transcription in kinetoplastid protozoa: why be normal?

Transcription in the kinetoplastid protozoa shows substantial variation from the paradigms of eukaryotic gene expression, including polycistronic transcription, a paucity of RNA polymerase (RNAP) II promoters, no qualitative regulated transcription initiation for most protein-coding genes, transcription of some protein-coding genes by RNAP I, an exclusive subnuclear location for VSG transcription, the dependence of small nuclear RNA gene transcription on an upstream tRNA gene, and the synthesis of mitochondrial tRNAs in the nucleus. Here, we present a broad overview of what is known about transcription in the kinetoplastids and what has yet to be determined.

Antigenic variation and the African trypanosome genome

African trypanosomes are protozoan parasites that reside in the mammalian bloodstream where they constantly confront the immune responses directed against them. They keep one-step-ahead of the immune system by continually switching from the expression of one variant surface glycoprotein (VSG) on their surface to the expression of another immunologically distinct VSG-a phenomenon called antigenic variation. About 1000 VSG genes (VSGs) and pseudo-VSGs are scattered throughout the trypanosome genome, all of which are transcriptionally silent except for one. Usually, the active VSG has been recently duplicated and translocated to one of about 20 potential bloodstream VSG expression sites (B-ESs). Each of the 20 potential B-ESs is adjacent to a chromosomal telomere, but only one B-ES is actively transcribed in a given organism. Recent evidence suggests the active B-ES is situated in an extra-nucleolar body of the nucleus where it is transcribed by RNA polymerase I. Members of another group of about 20 telomere-linked VSG expression sites (the M-ESs) are expressed only during the metacyclic stage of the parasite in its tsetse fly vector. Progress in sequencing the African trypanosome genome has led to additional insights on the organization of genes within both groups of ESs that may ultimately suggest better ways to control or eliminate this deadly pathogen.

Increased trypanolytic activity in sera of sleeping sickness patients after chemotherapy

We tested sera from patients previously treated for human African trypanosomiasis, from patients infected with trypanosomes, and from individuals never diagnosed with African trypanosomiasis living in the Trypanosoma brucei gambiense sleeping sickness focus of Mbini in Equatorial Guinea for their trypanolytic activity against bloodstream forms of T. b. rhodesiense expressing a metacyclic and bloodstream variant surface glycoprotein (VSG). Nearly 80% of the sera from treated patients showed high trypanolytic activity against trypanosomes expressing a metacyclic VSG. The trypanolytic activity of part of these sera was mediated by IgM while that of the other part was antibody-independent. On the other hand, only 40% of the sera exhibited high trypanolytic activity against trypanosomes expressing a bloodstream VSG which also was almost completely abolished by heat-inactivation. In contrast, most sera from infected and negative individuals displayed only low to moderate trypanolytic activity against either trypanosomes expressing a metacyclic or a bloodstream VSG. These results suggest that trypanolytic activity of sera increases after African sleeping sickness and is directed against trypanosomes expressing metacyclic VSG.

Control of VSG gene expression sites

Trypanosoma brucei survives in mammals by antigenic variation of its surface coat consisting of variant surface glycoprotein (VSG). Trypanosomes change coat mainly by replacing the transcribed VSG genes in an active telomeric expression site by a different VSG gene. There are about 20 different expression sites and trypanosomes can also change coat by switching the site that is active. This review summarizes recent work on the mechanism of site switching and on the way inactive expression sites are kept silent.

Conservation of metacyclic variant surface glycoprotein expression sites among different trypanosome isolates

We identified in a Trypanosoma brucei brucei strain (AnTat 1) an expression site for a metacyclic variant surface glycoprotein (MVSG) gene (MVSG) that was previously characterized in a T. b. rhodesiense strain (WRATat 1.1). The 3.4 kb sequences of the two expression sites are 99.6% identical, with no differences in the sequence of the 1.5 kb MVSG. Two other MVSGs in the WRATat 1.1 genome are not present in the AnTat 1 genome. In addition, five other T. b. brucei and T. b. rhodesiense strains, isolated in the same geographic region as the two former strains, do not contain any of these three MVSGs. Two of these five strains, however, appear to possess a very similar MVSG expression site, but with different MVSGs in it. Thus, the presence of the same MVSG in the same expression site in two different isolates is unusual and may be the result of genetic exchange in the field between T. b. brucei and T. b. rhodesiense isolates. Analysis of other African trypanosome strains for the presence of the three WRATat 1.1 MVSG expression sites demonstrated that the expression sites' promoter sequences are much more likely to be present than are specific MVSGs, suggesting that loss of MVSGs is the result of replacement by other VSGs. The promoter region of the MVSG expression site active in the WRATat 1.1 MVAT7 variant was found to be highly conserved among T. b. brucei, T. b. rhodesiense and T. b. gambiense group 2 isolates, whereas it does not occur in the T. b. gambiense group 1 isolates tested. A phylogenetic analysis of this promoter region sequence shows that the T. b. gambiense group 2 isolates form a monophyletic clade well separated from the T. b. brucei/T. b. rhodesiense isolates. Thus, whilst the T. b. brucei, T. b. rhodesiense and T. b. gambiense group 2 isolates are closely related but heterogenous, molecular tools may be developed to distinguish T. b. gambiense group 2 isolates from the others.

Search for promoters for the GARP and rRNA genes of Trypanosoma congolense

A search was conducted for transcriptional promoters in Trypanosoma congolense. A promoter test plasmid was constructed utilising the luciferase coding region flanked by the intergenic regions of a T. congolense gene encoding GARP, the glutamic acid and alanine rich protein on the surface of procyclic organisms. Using this plasmid, sequences located upstream of an 18S rRNA gene were tested in transient transfection assays for their ability to promote luciferase expression. A rRNA promoter fragment of 377 bp was identified that increases luciferase activity by as much as 35,000-fold above background levels. The rRNA transcription initiation site is located 961 bp upstream of the 18S rRNA gene and immediately downstream of 6 bp imperfect repeats. The plasmid was also used to examine sequences upstream of a GARP gene cluster in two different T. congolense strains for promoter activity. In contrast to the findings of another group, we were unable to detect promoter activity upstream of these GARP genes in either strain. We conclude that the GARP gene promoter, if it exists, has less than 0.03% (1/3000) of the activity of the rRNA promoter in this luciferase-based assay.

A structural and transcription pattern for variant surface glycoprotein gene expression sites used in metacyclic stage Trypanosoma brucei

African trypanosomes first express the variant surface glycoprotein (VSG) at the metacyclic stage in the tsetse fly vector, in preparation for transfer into the mammal. Metacyclic (M)VSGs comprise a specific VSG repertoire subset and their expression is regulated differently from that of bloodstream VSGs, involving exclusively transcriptional regulation during the life cycle. To identify basic structural and functional features that may be common to MVSG telomeric transcription units, we have characterized the anatomy and transcription of the telomere containing the ILTat 1.61 MVSG gene. This telomere contains pseudogenes of the ESAG1 and ESAG9 families found in bloodstream VSG transcription units. The 1.61 MVSG occupies a monocistronic transcription unit and is transcriptionally controlled through the life cycle. The 1.61, and also the 1.22, MVSG transcription initiation site sequences resemble eukaryotic initiator elements. Sequence comparison reveals that four out of five characterized MVSG expression sites have a conserved region 2.0-4.7 kb long upstream of the MVSG. In some cases, this region contains not only the transcription initiation site that we have observed to be active in fly-transmitted trypanosomes but also, upstream, another sequence, described elsewhere as a 'putative promoter' for the MVAT set of M/VSGs (Nagoshi YL, Alarcon CM, Donelson JE. A monocistronic transcript for a trypanosome variant surface glycoprotein, Mol Biochem Parasitol 1995;72:33-45). In fly-transmitted trypanosomes, the latter element is transcriptionally silent. Our analysis of the structure of MVSG telomeres suggests that metacyclic expression sites arose from bloodstream expression sites.

Transcription of 'inactive' expression sites in African trypanosomes leads to expression of multiple transferrin receptor RNAs in bloodstream forms

African trypanosomes express a heterodimeric transferrin receptor that mediates iron uptake from the host bloodstream. The genes encoding the receptor, ESAG6 and ESAG7, are found at the beginning of VSG expression sites: these are telomeric, polycistronic transcription units that each terminate with a gene encoding a trypanosome variant surface glycoprotein, VSG. Approximately 20 of these VSG expression sites are found in the trypanosome genome, but only one VSG is expressed at a time. The conventional view is that one expression site promoter is extremely active whereas the others are either inactive or show very low, poorly processive activity, and that all transferrin receptor molecules are encoded by the active expression site. The 3'-end of the ESAG6 gene is more than 5 kb from the promoter. We show here that 20% of ESAG6 mRNA originates from the 'inactive' expression sites. We suggest that many expression site promoters in trypanosomes show low-level activity throughout the life cycle, and that transcription proceeds for at least 5 kb. This suggests a simplified model of VSG expression site control, whereby the only regulated event is the strong activation of a single expression site promoter in bloodstream forms.

Leaky transcription of variant surface glycoprotein gene expression sites in bloodstream african trypanosomes

Trypanosoma brucei undergoes antigenic variation by periodically switching the expression of its variant surface glycoprotein (VSG) genes (vsg) among an estimated 20-40 telomere-linked expression sites (ES), only one of which is fully active at a given time. We found that in bloodstream trypanosomes one ES is transcribed at a high level and other ESs are expressed at low levels, resulting in organisms containing one abundant VSG mRNA and several rare VSG RNAs. Some of the rare VSG mRNAs come from monocistronic ESs in which the promoters are situated about 2 kilobases upstream of the vsg, in contrast to the polycistronic ESs in which the promoters are located 45-60 kilobases upstream of the vsg. The monocistronic ES containing the MVAT4 vsg does not include the ES-associated genes (esag) that occur between the promoter and the vsg in polycistronic ESs. However, bloodstream MVAT4 trypanosomes contain the mRNAs for many different ESAGs 6 and 7 (transferrin receptors), suggesting that polycistronic ESs are partially active in this clone. To explain these findings, we propose a model in which both mono- and polycistronic ESs are controlled by a similar mechanism throughout the parasite's life cycle. Certain VSGs are preferentially expressed in metacyclic versus bloodstream stages as a result of differences in ESAG expression and the proximity of the promoters to the vsg and telomere.

The anatomy and transcription of a monocistronic expression site for a metacyclic variant surface glycoprotein gene in Trypanosoma brucei

African trypanosomes evade the immune response of their mammalian hosts by switching the expression of their variant surface glycoprotein genes (vsg). The bloodstream trypanosome clone MVAT4 of Trypanosoma brucei rhodesiense expresses a metacyclic vsg as a monocistronic RNA from a promoter located 2 kilobases (kb) upstream of its start codon. Determination of 23 kb of sequence at the metacyclic variant antigen type 4 (MVAT) vsg expression site (ES) revealed an ES-associated gene (esag) 1 preceded by an ingi retroposon and an inverted region containing an unrelated vsg, short stretches of 70-bp repeats and a pseudo esag 3. Nuclear run-on experiments indicate that the 18-kb region upstream of the MVAT4 vsg promoter is transcriptionally silent. However, multiple members of different esag families are expressed from elsewhere in the genome. The MVAT4 vsg promoter is highly repressed in the procyclic stage, in contrast to the known polycistronic vsg ESs which undergo abortive transcription. Activation of the MVAT4 vsg ES occurs in situ without nucleotide sequence changes, although this monocistronic ES undergoes a pattern of base J modifications similar to that reported for the polycistronic ESs. The relative simplicity of the MVAT4 vsg ES and the uncoupled expression of the vsg and esags provide a unique opportunity for investigating the molecular mechanisms responsible for antigenic variation in African trypanosomes.

Control of gene expression in Trypanosomatidae

The study of mechanisms which control gene expression in trypanosomatids has developed at an increasing rate since 1989 when the first successful DNA transfection experiments were reported. Using primarily Trypanosoma brucei as a model, several groups have begun to elucidate the basic control mechanisms and to define the cellular factors involved in mRNA transcription, processing and translation in these parasites. This review focuses on the most recent studies regarding a subset of genes that are expressed differentially during the life cycle of three groups of parasites. In addition to T. brucei, I will address studies on gene regulation in a few species of Leishmania and the results obtained by a much more limited group of laboratories studying gene expression in Trypanosoma cruzi. It is becoming evident that the regulatory strategies chosen by different species of trypanosomatids are not similar, and that for these very successful parasites it is probably advantageous to employ multiple mechanisms simultaneously. In addition, with the increasing numbers of parasite genes that have now been submitted to molecular dissection, it is also becoming evident that, among the various strategies for gene expression control, there is a predominance of regulatory pathways acting at the post-transcriptional level.

Generation of expressed sequence tags as physical landmarks in the genome of Trypanosoma brucei

Previous molecular genetic studies on the African trypanosome have focused on only a few genes and gene products, the majority of which are concerned with surface antigenic variation; consequently, an insignificant number of the genes of this organism have been characterized to date. In order to: (1) identify new genes and analyze their expression profile, (2) generate expressed sequence tags (ESTs) for derivation of a physical map of the trypanosome genome, and (3) make available the partial sequence information and the corresponding clones for general biomedical research on the parasite, we have performed single-pass sequencing of random, directionally cloned cDNAs from a bloodstream form Trypanosoma brucei rhodesiense library. Analysis of 2128 such ESTs sequenced so far in this study showed significant similarities [BLASTX P(n)-value < 10(-4), and a match > 10 amino acid residues] with proteins whose genes have been described in diverse organisms including man, rodents, kinetoplastids, yeasts and plants. A number of the ESTs encode homologues of proteins involved in various functions including signal reception and transduction, cell division, gene regulation, DNA repair and replication, general metabolism, and structural integrity. Although some of these genes may have been expected to be present in the African trypanosomes, the majority of them had not previously been described in these organisms. A large proportion, 768 individual ESTs (36%, representing 385 different transcripts), had a significant homology with genes described in organisms other than the African trypanosomes; however, 15% of the ESTs were from genes already described in trypanosomes. Among the ESTs analysed were 462 distinct known genes, only 77 of which have been described in T. brucei. Approximately 52% of the ESTs did not show any significant homology with the sequences in any of the public domain databases.

Testing promoter activity in the trypanosome genome: isolation of a metacyclic-type VSG promoter, and unexpected insights into RNA polymerase II transcription

In trypanosomes, most genes are arranged in polycistronic transcription units. Individual mRNAs are generated by 5'-trans splicing and 3' polyadenylation. Remarkably, no regulation of RNA polymerase II transcription has been detected although many RNAs are differentially expressed during kinetoplastid life cycles. Demonstration of specific class II promoters is complicated by the difficulty in distinguishing between genuine promoter activity and stimulation of trans splicing. Using vectors that were designed to allow the detection of low promoter activities in a transcriptionally silent chromosomal context, we isolated a novel trypanosome RNA polymerase I promoter. We were however unable to detect class II promoter activity in any tested DNA fragment. We also integrated genes which were preceded by a T3 promoter into the genome of cells expressing bacteriophage T3 polymerase: surprisingly, transcription was alpha-amanitin sensitive. One possible interpretation of these results is that in trypanosomes, RNA polymerase II initiation is favored by genomic accessibility and double-strand melting.

Changing the end: antigenic variation orchestrated at the telomeres of African trypanosomes

African trypanosomes express the gene encoding their variant surface glycoprotein (VSG) surface coat from one of many telomeric expression sites. This genomic location at chromosome ends not only allows easy exchange of VSG gene cassettes using various mechanisms of DNA recombination but also appears to play a role in VSG gene expression site control.

African trypanosomes have differentially expressed genes encoding homologues of the Leishmania GP63 surface protease

The genomes of various Leishmania parasites contain tandemly arrayed genes encoding an abundant 63-kDa surface glycoprotein called GP63. Leishmania GP63s are metalloproteases that play an important role in the invasion and survival of the parasites within the macrophage, and their presence on the Leishmania surface has been correlated with resistance to complement-mediated lysis. Here we report the identification of GP63-like genes in African trypanosomes. The predicted trypanosome and Leishmania GP63s share a metalloprotease catalytic site motif of HEXXH as well as 19 cysteines and 10 prolines, implying a conservation of enzymatic activity and secondary/tertiary structure. The trypanosome GP63 genes are transcribed equally in procyclic and bloodstream trypanosomes, but their mRNAs accumulate to a 50-fold higher steady state level in bloodstream trypanosomes, where the ratio of mRNAs for GP63 and variant surface glycoprotein is about 1:150. Transcription of the GP63 genes is sensitive to alpha-amanitin, indicating that they are transcribed by a different polymerase than the variant surface glycoprotein genes. These results lead to a reconsideration of the potential functions of GP63, inasmuch as African trypanosomes are not known to interact with macrophages and do not have an intracellular stage during their life cycle.

Binding of trans-acting factors to the double-stranded variant surface glycoprotein (VSG) expression site promoter of Trypanosoma brucei

Trypanosoma brucei evades its host's immune response by utilizing the system of antigenic variation, whereby the organism sequentially expresses antigenically distinct variant surface glycoproteins (VSGs). Actively expressed VSG genes are found in VSG expression sites (ESs), and transcription of these ESs is directed by a small promoter composed of two essential cis-acting elements, the VSG ES promoter upstream element (VUE) and VSG ES promoter downstream element (VDE). Using electrophoretic mobility shift assays, we have identified double-stranded DNA binding activity in bloodstream-form trypanosome nuclear extracts. This activity, the VEP complex, is specific for the VSG ES promoter, and requires the intact sequences of the VUE and VDE in the appropriate spacing. These requirements of VEP Complex formation parallel the requirements for promoter function, suggesting that the VEP complex may be composed of functionally significant trans-acting factors. Furthermore, the requirement of both elements suggests that the binding of factors to the promoter may be cooperative. However, subtly different binding characteristics were observed when we used nuclear extracts derived from procyclic trypanosomes.

Co-duplication of a variant surface glycoprotein gene and its promoter to an expression site in African trypanosomes

Activation of the metacyclic variant antigen type 7 (MVAT7) variant surface glycoprotein (VSG) gene in bloodstream Trypanosoma brucei rhodesiense involves a duplicative transposition of the gene. The DNA transposition unit extends from a site approximately 3.0 kilobases upstream of the VSG gene through the coding region and includes a 73-base pair sequence that possesses promoter activity in transient transfections. This MVAT7 promoter has 80% identity to a previously characterized promoter for the MVAT4 VSG gene. Nuclear run-on assays demonstrate that the MVAT7 promoter is active in MVAT7 bloodstream organisms and that its transcript is synthesized by an RNA polymerase resistant to alpha-amanitin, consistent with previously published reports regarding VSG gene transcription. The transcription start site was identified by primer extension studies and a modified rapid amplification of cDNA ends protocol. Selective mutational analysis of the MVAT7 promoter showed that two conserved trinucleotide regions are important for full promoter function. This study demonstrates that the MVAT7 VSG gene is co-duplicated with its promoter and transcribed into a monocistronic precursor RNA.

The gene for a T lymphocyte triggering factor from African trypanosomes

An early and essential event in the protective immune response against most viruses and protozoa is the production of interferon-gamma (IFN-gamma). In contrast, during infection with African trypanosomes, protozoan parasites that cause human sleeping sickness, the increased levels of IFN-gamma do not correlate with a protective response. We showed previously that African trypanosomes express a protein called T lymphocyte triggering factor (TLTF), which triggers CD8(+) T lymphocytes to proliferate and to secrete IFN-gamma. Here, we isolate the gene for TLTF and demonstrate that the recombinant version of TLTF specifically induces CD8(+), but not CD4(+), T cells to secrete IFN-gamma. Studies with TLTF fused to the green fluorescent protein show that TLTF is localized to small vesicles that are found primarily at or near the flagellar pocket, the site of secretion in trypanosomes. TLTF is likely to be only the first example of a class of proteins that we designate as trypanokines, i.e., factors secreted by trypanosomes that modulate the cytokine network of the host immune system for the benefit of the parasite.

Manipulation of the vsg co-transposed region increases expression-site switching in Trypanosoma brucei

Disruption of a region of DNA in Trypanosoma brucei immediately upstream of the expressed telomere-proximal variant surface glycoprotein gene (vsg), known as the co-transposed region (CTR), can cause a dramatic increase in the rate at which the active expression site (ES) is switched off and a new ES is switched on. Deletion of most of the CTR in two ESs caused a greater than 100-fold increase in the rate of ES switching, to about 1.3 x 10(-4) per generation. A more dramatic effect was observed when the entire CTR and the 5' coding region of the expressed vsg221 were deleted. In this case a new ES was activated within a few cell divisions. This switch also occurred in cell lines where a second vsg had been inserted into the ES, prior to CTR deletion. These cell lines, which stably co-expressed the inserted and endogenous Vsgs, in equal amounts, did not differ from the wild-type in growth rate or switching frequency, suggesting that simultaneous expression of two Vsgs has no intrinsic effect. CTR deletion did not disturb the inserted vsg117. We tentatively conclude that it was not the disruption of the vsg221 in itself that destabilized the ES. All of the observed switches occurred without additional detectable DNA rearrangements in the switched ES. Deletion of the 70-bp repeats and/or a vsg pseudogene upstream of the CTR did not affect ES stability. Several speculative interpretations of these observation are offered, the most intriguing of which is that the CTR plays some role in modulating chromatin conformation at an ES.

A survey of the Trypanosoma brucei rhodesiense genome using shotgun sequencing

A comparison of the efficiency of sequencing random genomic DNA fragments versus random cDNAs for the discovery of new genes in African trypanosomes was undertaken. Trypanosome DNA was sheared to a 1.5-2.5 kb size distribution, cloned into a plasmid and the sequences at both ends of 183 cloned fragments determined. Sequences of both kinetoplast and nuclear DNA were identified. New coding regions were discovered for a variety of proteins, including cell division proteins, an RNA-binding protein and a homologue of the Leishmania surface protease GP63. In some cases, each end of a fragment was found to contain a different gene, demonstrating the proximity of those genes and suggesting that the density of genes in the African trypanosome genome is quite high. Repetitive sequence elements found included telomeric hexamer repeats, 76 bp repeats associated with VSG gene expression sites, 177 bp satellite repeats in minichromosomes and the Ingi transposon-like elements. In contrast to cDNA sequencing, no ribosomal protein genes were detected. For the sake of comparison, the sequences of 190 expressed sequence tags (ESTs) were also determined, and a similar number of new trypanosomal homologues were found including homologues of another putative surface protein and a human leucine-rich repeat-containing protein. We conclude from this analysis and our previous work that sequencing random DNA fragments in African trypanosomes is as efficient for gene discovery as is sequencing random cDNA clones.

Is point mutagenesis a mechanism for antigenic variation in Trypanosoma brucei?

Antigenic variation in African trypanosomes proceeds by switching between different variant surface glycoprotein (VSG) molecules, whose extensive epitope differences enable evasion of antibody responses. Each trypanosome has approximately 1000 basic copy VSG genes inside chromosomes and a subset located at telomeres. Switching usually involves different individual basic copy genes being duplicated, as an expression linked copy, into a transcriptionally active site. In a few cases expression linked copies with a number of point mutations have been observed, leading to the suggestion that point mutagenesis provides another mechanism of antigenic variation. The most extensive example is a VSG gene that is normally activated in the metacyclic population in the tsetse fly, but the point mutations were detected in expression linked copies generated during bloodstream infection, after prolonged growth and selection. It was suggested that particularly telomeric or metacyclic VSG genes might undergo point mutagenesis during expression linked copy formation. To test this we have cloned 3 trypanosomes very soon after they had generated, during mouse infection, expression linked copies of the metacyclic VSG gene ILTat 1.22 and have detected only a single point mutation which is present in one expression linked copy, but not the corresponding basic copy, gene. This mutation does not prevent binding of a neutralizing antibody. Extensive VSG gene point mutagenesis may be a consequence merely of prolonged growth and extensive selection. There is not a single reported case of a point mutated VSG presenting a completely new set of exposed epitopes, suggesting point mutagenesis is unlikely to be an authentic mechanism for antigenic variation.

Analysis of a hybrid PARP/VSG ES promoter in procyclic trypanosomes

The parasite Trypanosoma brucei changes its variant surface glycoprotein (VSG) coat to escape the host immune system. At a chromosomal locus, we analyzed the promoter that controls expression of VSG genes, using a system developed in collaboration with Urményi and Van der Ploeg (Urményi, T.P. and Van der Ploeg, L.H.T. (1995) Nucleic Acids Res. 23,1010-1016), and showed that the variant surface glycoprotein expression site (VSG ES) promoter directed < 6% the CAT activity produced by the procyclic acidic repetitive protein (PARP) promoter at the same locus. We identified a fragment from the PARP promoter (bp -743 to -111) that contained no intrinsic promoter activity. However, when this fragment was cloned 5' to 3' upstream of the VSG ES promoter, and this hybrid PARP/VSG ES promoter was stably integrated at the RNA polymerase (Pol) II largest subunit gene locus, expression from a CAT gene cassette increased 10-fold. Nascent RNA analysis independently showed that the relative efficiency of alpha-amanitin-resistant transcription directed by the hybrid PARP/VSG ES promoter was more than 6-fold higher than that directed by the wild-type VSG ES promoter. Furthermore, using nascent RNA protection assays, we mapped the transcription start site of the hybrid PARP/VSG ES promoter to the same initiation site as that of the wild-type VSG ES promoter. Finally, we evaluated the functional activity of the hybrid PARP/VSG ES mutant promoter at the dominant VSG gene expression site on the 1.5-Mb chromosome. At this locus, as well, the hybrid PARP/VSG ES promoter directed almost 3-times as much CAT activity as that of the wild-type VSG ES promoter.

The promoter for the ribosomal RNA genes of Leishmania chagasi

A promoter for the rRNA genes of Leishmania chagasi was found to be located about 1 kb upstream of the 18S rRNA coding region and immediately downstream of 64 bp tandem repeats. Its approximate boundaries and corresponding transcription start site were determined by transient transfections and primer extension assays. This promoter for RNA polymerase I has differing activities when transfected into various Leishmania species and no activity in Trypanosoma cruzi. Its sequence has no obvious similarities with other known rRNA promoters in Trypanosomatids. Depending on the species, this promoter can be used to increase expression of a protein from a plasmid in Leishmania by as much as 45-fold over that from a plasmid lacking a promoter.

Differential expression of the expression site-associated gene I family in African trypanosomes

A minimum of 20 different mRNA species encoding related members of the expression site-associated gene I (ESAG-I) family occur in metacyclic variant antigen type 4 bloodstream trypanosomes. None of these ESAG-I mRNAs are derived from the metacyclic variant antigen type 4 variant surface glycoprotein (VSG) gene expression site, and some appear to come from pseudogenes. The ESAG-Is are transcribed in both procyclic and bloodstream trypanosomes, but their mRNAs accumulate to a detectable steady state level only in bloodstream trypanosomes. At least five different groups of 3'-untranslated regions (3'-UTRs) are represented among these ESAG-I mRNAs, suggesting that the 3'-UTR does not contribute to their differential expression. Some ESAG-I mRNAs completely lack a 3'-UTR or have only a single nucleotide as a 3'-UTR. Transcription of the ESAG-Is is sensitive to alpha-amanitin, indicating that they are transcribed by a different RNA polymerase than the VSG genes. These results collectively demonstrate that ESAG-I's are a heterogeneous population that can be expressed independently of VSG genes, but like the VSG genes, their mRNAs are present in the bloodstream stage of the parasite and not in the procyclic stage.

Antigenic variation in trypanosomes: secrets surface slowly

Among pathogenic micro-organisms that evade the mammalian immune responses. Trypanosoma brucei has developed the most elaborate capacity for antigenic variation. Trypanosomes branched early during eukaryotic evolution. They are characterized by many aberrations, ranging from the unusual compartmentation of metabolic pathways to the heresy of RNA editing. The ubiquitous phenomenon of glycosylphosphatidylinositol-anchoring of eukaryotic plasma membrane proteins and RNA trans-splicing (trypanosome genes contain no introns), which adds an identical leader sequence to all trypanosome mRNAs, were first defined during studies of antigenic variation. Genetic transformation of trypanosomes and the high efficiency of gene targeting provide new opportunities to investigate the regulation of antigenic variation. There is every reason to expect trypanosomes to provide further surprises and insights into the evolution of genetic regulatory mechanisms.

A detailed mutational analysis of the VSG gene expression site promoter

The African trypanosome Trypanosoma brucei is a protozoan parasite that causes the disease African sleeping sickness. The parasite avoids the host's immune response by the process of antigenic variation, or by sequentially expressing antigenically different cell-surface coat proteins. These proteins, called variant surface glycoproteins (VSGs), are expressed from a specific locus, the VSG gene expression site (ES). In an attempt to understand expression of VSG genes, we expanded on earlier investigations of the promoter that controls the large VSG gene expression site transcription unit. We studied VSG ES promoter function both in transient transfection assays, and after stable integration at a chromosomal locus. Analysis of closely spaced deletion mutants showed that the minimum VSG ES promoter fragment that gives full activity is extremely small, and mapped precisely to a fragment that contains no more than -67 bp 5' to the putative transcription initiation site. The promoter lacked an upstream control element, or UCE, an element found at the PARP promoter, and at most eukaryotic Pol I promoters. Furthermore, linker scanning mutagenesis demonstrated that the VSG ES promoter contains at least two essential regulatory elements, including sequences within the region -67/-60 and the region -35/-20, both numbered relative to the initiation site. An altered promoter with mutated nucleotides surrounding the transcription initiation site still directed wild-type levels of expression. In this study, the results were similar for both insect and bloodstream form trypanosomes, suggesting that the same basic machinery for expression from the VSG ES promoter is found in both stages of the parasite.

A developmentally regulated position effect at a telomeric locus in Trypanosoma brucei

Trypanosoma brucei undergoes antigenic variation in the mammalian host. This can be achieved by activation and inactivation of telomeric variant-specific surface glycoprotein genes (vsg). In procyclic (insect midgut stage) cells, Vsg is not expressed. The mechanisms that regulate transcription of vsg expression sites (ESs) are unknown. Here we demonstrate that transcription from three different promoters was repressed when they were inserted at a transcriptionally silent telomere-proximal locus in bloodstream-form cells. This position effect was stable and heritable. Only transcription from an ES promoter was repressed in procyclic cells. The observed position effect and the promoter-specific developmental regulation suggest that these phenomena reflect the mechanisms that regulate vsg expression.

A ribosomal DNA promoter replacing the promoter of a telomeric VSG gene expression site can be efficiently switched on and off in T. brucei

Trypanosoma brucei survives in the mammalian blood-stream by regularly changing its variant surface glycoprotein (VSG) coat. The active VSG gene is located in a telomeric expression site, and coat switching occurs either by replacing the transcribed VSG gene or by changing the expression site that is active. To determine whether VSG expression site control requires promoter-specific sequences, we replaced the active VSG expression site promoter in bloodstream-form T. brucei with a ribosomal DNA (rDNA) promoter. These transformants were fully infective in laboratory animals, and the rDNA promoter, which is normally constitutively active, was efficiently inactivated and reactivated in the context of the VSG gene expression site. As there is no sequence similarity between the VSG expression site promoter and the rDNA promoter, VSG expression site control does not involve sequences specific to the VSG expression site promoter. We conclude that an epigenetic mechanism, such as telomeric silencing, is involved in VSG expression site control in bloodstream-form T. brucei.

cDNA expressed sequence tags of Trypanosoma brucei rhodesiense provide new insights into the biology of the parasite

A total of 518 expressed sequence tags (ESTs) have been generated from clones randomly selected from a cDNA library and a spliced leader sub-library of a Trypanosoma brucei rhodesiense bloodstream clone. 205 (39%) of the clones were identified based on matches to 113 unique genes in the public databases. Of these, 71 cDNAs display significant similarities to genes in unrelated organisms encoding metabolic enzymes, signal transduction proteins, transcription factors, ribosomal proteins, histones, a proliferation-associated protein and thimet oligopeptidase, among others. 313 of the cDNAs are not related to any other sequences in the databases. These cDNA ESTs provide new avenues of research for exploring both the novel trypanosome-specific genes and the genome organization of this parasite, as well as a resource for identifying trypanosome homologs to genes expressed in other organisms.

The putative promoter for a metacyclic VSG gene in African trypanosomes

During their metacyclic developmental stage, African trypanosomes are coated with one of 12-15 variant surface glycoproteins (VSGs) that define different metacyclic variant antigen types (MVATs). The MVAT VSG genes are located near telomeres of large chromosomes and are expressed without rearrangement in the metacyclic stage. We have cloned and examined the telomere-linked MVAT5 VSG gene and its upstream expression site associated gene (ESAG I) which are separated by 4.5 kb. Within this 4.5-kb intergenic region is an 87-bp sequence that serves as a strong promoter for a luciferase reporter gene in transient transfection assays. This 87-bp sequence is similar, but not identical, to the promoter for another MVAT VSG gene. UV irradiation experiments were used to detect RNA synthesis from this MVAT5 promoter in bloodstream trypanosomes expressing an unrelated VSG. We propose that this sequence is a specific promoter for the MVAT5 VSG mRNA that occurs in about 10% of the trypanosome population during the metacyclic stage of the parasites' life cycle.

Mechanisms of stage-regulated gene expression in kinetoplastida

During their life cycle, trypanosomatid parasites of mammals encounter substantially different environments in their hosts and insect vectors, to which they must adapt by undergoing a series of differentiation processes. At the molecular level, these processes must be the direct result of an elaborate series of changes in stage-regulated expression of a wide range of gene products. How are these changes accomplished? In this review, Sheila Graham discusses some recent advances in understanding the mechanisms of gene expression in trypanosomatids, and examines some clues to some intriguingly complex means of regulating life cycle stage-specific gene expression.