High homology between variant surface glycoprotein gene expression sites of Trypanosoma brucei and Trypanosoma gambiense

Conserved sequence of the TgsGP gene in Group 1 Trypanosoma brucei gambiense

The trypanosome responsible for the majority of cases of human trypanosomiasis in Africa is Group 1 Trypanosoma brucei gambiense. Currently the most reliable test for the parasite is based on a single gene, which encodes a 47kDa receptor-like T. b. gambiense-specific glycoprotein, TgsGP, expressed in the flagellar pocket of bloodstream forms. Although TgsGP has been demonstrated in T. b. gambiense throughout its geographic range, similar genes have been demonstrated in other T. brucei sspp. isolates, and there are no data on the extent of sequence variation in TgsGP. Here we have carried out a comparison of TgsGP sequences in a range of Group 1 T. b. gambiense isolates and compared the gene to homologues in other T. brucei sspp. in order to provide information to support the use of this gene as the key identification target for Group 1 T. b. gambiense. We demonstrate that the sequence of TgsGP is well conserved in Group 1 T. b. gambiense across the endemic range of gambian human trypanosomiasis and confirm that this gene is a suitable target for specific detection of this parasite. The TgsGp-like genes in some isolates of T. b. brucei, T. b. rhodesiense and Group 2 T. b. gambiense are closely similar to VSG Tb10.v4.0178, which may be the ancestral gene from which TgsGP was derived.

Isolation and analysis of the genetic diversity of repertoires of VSG expression site containing telomeres from Trypanosoma brucei gambiense, T. b. brucei and T. equiperdum

Background

African trypanosomes (including Trypanosoma brucei) are unicellular parasites which multiply in the mammalian bloodstream. T. brucei has about twenty telomeric bloodstream form Variant Surface Glycoprotein (VSG) expression sites (BESs), of which one is expressed at a time in a mutually exclusive fashion. BESs are polycistronic transcription units, containing a variety of families of expression site associated genes (ESAGs) in addition to the telomeric VSG. These polymorphic ESAG families are thought to play a role in parasite-host adaptation, and it has been proposed that ESAG diversity might be related to host range. Analysis of the genetic diversity of these telomeric gene families has been confounded by the underrepresentation of telomeric sequences in standard libraries. We have previously developed a method to selectively isolate sets of trypanosome BES containing telomeres using Transformation associated recombination (TAR) cloning in yeast.

Results

Here we describe the isolation of repertoires of BES containing telomeres from three trypanosome subspecies: Trypanosoma brucei gambiense DAL 972 (causative agent of West-African trypanosomiasis), T. b. brucei EATRO 2340 (a nonhuman infective strain) and T. equiperdum STIB 818 (which causes a sexually transmitted disease in equines). We have sequenced and analysed the genetic diversity at four BES loci (BES promoter region, ESAG6, ESAG5 and ESAG2) from these three trypanosome BES repertoires.

Conclusion

With the exception of ESAG2, the BES sequence repertoires derived from T. b. gambiense are both less diverse than and nearly reciprocally monophyletic relative to those from T. b. brucei and T. equiperdum. Furthermore, although we find evidence for adaptive evolution in all three ESAG repertoires in T. b. brucei and T. equiperdum, only ESAG2 appears to be under diversifying selection in T. b. gambiense. This low level of variation in the T. b. gambiense BES sequence repertoires is consistent both with the relatively narrow host range of this subspecies and its apparent long-term clonality. However, our data does not show a clear correlation between size of trypanosome host range and either number of BESs or extent of ESAG genetic diversity.

Human African Trypanosomiasis: Epidemiology and Control

Human African trypanosomiasis (HAT), or sleeping sickness, describes not one but two discrete diseases: that caused by Trypanosoma brucei rhodesiense and that caused by T. b. gambiense. The Gambian form is currently a major public health problem over vast areas of central and western Africa, while the zoonotic, Rhodesian form continues to present a serious health risk in eastern and southern Africa. The two parasites cause distinct clinical manifestations, and there are significant differences in the epidemiology of the diseases caused. We discuss the differences between the diseases caused by the two parasites, with an emphasis on disease burden, reservoir hosts, transmission, diagnosis, treatment and control. We analyse how these differences impacted on historical disease control trends and how they can inform contemporary treatment and control options. We consider the optimal ways in which to devise HAT control policies in light of the differing biology and epidemiology of the parasites, and emphasise, in particular, the wider aspects of control policy, outlining the responsibilities of individuals, governments and international organisations in control programmes.

Apolipoprotein L-I is the trypanosome lytic factor of human serum

Human sleeping sickness in east Africa is caused by the parasite Trypanosoma brucei rhodesiense. The basis of this pathology is the resistance of these parasites to lysis by normal human serum (NHS). Resistance to NHS is conferred by a gene that encodes a truncated form of the variant surface glycoprotein termed serum resistance associated protein (SRA). We show that SRA is a lysosomal protein, and that the amino-terminal alpha-helix of SRA is responsible for resistance to NHS. This domain interacts strongly with a carboxy-terminal alpha-helix of the human-specific serum protein apolipoprotein L-I (apoL-I). Depleting NHS of apoL-I, by incubation with SRA or anti-apoL-I, led to the complete loss of trypanolytic activity. Addition of native or recombinant apoL-I either to apoL-I-depleted NHS or to fetal calf serum induced lysis of NHS-sensitive, but not NHS-resistant, trypanosomes. Confocal microscopy demonstrated that apoL-I is taken up through the endocytic pathway into the lysosome. We propose that apoL-I is the trypanosome lytic factor of NHS, and that SRA confers resistance to lysis by interaction with apoL-I in the lysosome.

The architecture of variant surface glycoprotein gene expression sites in Trypanosoma brucei

Trypanosoma brucei evades the immune system by switching between Variant Surface Glycoprotein (VSG) genes. The active VSG gene is transcribed in one of approximately 20 telomeric expression sites (ESs). It has been postulated that ES polymorphism plays a role in host adaptation. To gain more insight into ES architecture, we have determined the complete sequence of Bacterial Artificial Chromosomes (BACs) containing DNA from three ESs and their flanking regions. There was variation in the order and number of ES-associated genes (ESAGs). ESAGs 6 and 7, encoding transferrin receptor subunits, are the only ESAGs with functional copies in every ES that has been sequenced until now. A BAC clone containing the VO2 ES sequences comprised approximately half of a 330 kb 'intermediate' chromosome. The extensive similarity between this intermediate chromosome and the left telomere of T. brucei 927 chromosome I, suggests that this previously uncharacterised intermediate size class of chromosomes could have arisen from breakage of megabase chromosomes. Unexpected conservation of sequences, including pseudogenes, indicates that the multiple ESs could have arisen through a relatively recent amplification of a single ES.

Control and function of the bloodstream variant surface glycoprotein expression sites in Trypanosoma brucei

African trypanosomes escape the host immune response through a periodical change of their surface coat made of one major type of protein, the variant surface glycoprotein. From a repertoire of a thousand variant surface glycoprotein genes available, only one is expressed at a time, and this takes place in a specialised expression site itself selected from a collection of an estimated 20-30 sites. As the specialised expression sites are long polycistronic transcription units, the variant surface glycoprotein is co-transcribed with several other genes termed expression site-associated genes. How do the trypanosomes only use a single specialised expression site at a time? Why are there two dozen specialised expression sites? What are the functions of the other genes of these transcription units? We review the currently available answers to these questions.

Molecular characterization of field isolates of human pathogenic trypanosomes

The accurate identification of each of the three subspecies of Trypanosoma brucei remains a challenging problem in the epidemiology of sleeping sickness. Advances in molecular characterization have revealed a much greater degree of heterogeneity within the species than previously supposed. Only group 1 T. b. gambiense stands out as a separate entity, defined by several molecular markers. T. b. rhodesiense is generally too similar to sympatric T. b. brucei strains to be distinguished from them by any particular molecular markers. Nevertheless, characterization of trypanosome isolates from humans and other animals has allowed the identification of potential reservoir hosts of T. b. rhodesiense. The recent discovery of a gene for human serum resistance may provide a useful marker for T. b. rhodesiense in the future. There have been few attempts to find associations between genetic markers and other biological characters, except human infectivity. However, virulence or fly transmissibility have been correlated with molecular markers in some instances.

The VSG expression sites of Trypanosoma brucei: multipurpose tools for the adaptation of the parasite to mammalian hosts

The variant surface glycoprotein (VSG) genes of Trypanosoma brucei are transcribed in telomeric loci termed VSG expression sites (ESs). Despite permanent initiation of transcription in most if not all of these multiple loci, RNA elongation is abortive except in bloodstream forms where full transcription up to the VSG occurs only in a single ES at a time. The ESs active in bloodstream forms are polycistronic and contain several genes in addition to the VSG, named ES-associated genes (ESAGs). So far 12 ESAGs have been identified, some of which are present only in some ESs. Most of these genes encode surface proteins and this list includes different glycosyl phosphatidyl inositol (GPI)-anchored proteins such as the heterodimeric receptor for the host transferrin (ESAG7/6), integral membrane proteins such as the receptor-like transmembrane adenylyl cyclase (ESAG4) and a surface transporter (ESAG10). An interesting exception is ESAG8, which may encode a cell cycle regulator involved in the differentiation of long slender into short stumpy bloodstream forms. Several ESAGs belong to multigene families including pseudogenes and members transcribed out of the ESs, named genes related to ESAGs (GRESAGs). However, some ESAGs (7, 6 and 8) appear to be restricted to the ESs. Most of these genes can be deleted from the active ES without apparently affecting the phenotype of bloodstream form trypanosomes, probably either due to the expression of ESAGs from 'inactive' ESs (ESAG7/6) or due to the expression of GRESAGs (in particular, GRESAGs4 and GRESAGs1). At least three ESAGs (ESAG7, ESAG6 and SRA) share the evolutionary origin of VSGs. The presence of these latter genes in ESs may confer an increased capacity of the parasite for adaptation to various mammalian hosts, as suggested in the case of ESAG7/6 and proven for SRA, which allows T. brucei to infect humans. Similarly, the existence of a collection of slightly different ESAG4s in the multiple ESs might provide the parasite with adenylyl cyclase isoforms that may regulate growth in response to different environmental conditions. The high transcription rate and high recombination level that prevail in VSG ESs may have favored the generation and/or recruitment in these sites of genes whose hyper-evolution allows adaptation to a larger variety of hosts.

A receptor-like flagellar pocket glycoprotein specific to Trypanosoma brucei gambiense

Trypanosoma brucei gambiense and T. b. rhodesiense are protozoan parasites causing sleeping sickness in humans due to their resistance to lysis by normal human serum (NHS). Based on the observation that the resistance gene of T. b. rhodesiense encodes a truncated form of the variant specific glycoprotein (VSG), we cloned a similar gene in T. b. gambiense using reverse transcription-linked polymerase chain reaction with VSG-specific primers. This gene, termed TgsGP for T. gambiense-specific glycoprotein, was found to be specific to T. b. gambiense. It is located close to a telomere and is transcribed by a pol II RNA polymerase, only at the bloodstream stage of the parasite development. TgsGP encodes a 47-kDa protein consisting of a N-terminal VSG domain presumably provided with a glycosylphosphatidylinositol (GPI) anchor sequence, similar to the pESAG6 subunit of the trypanosomal transferrin receptor. TgsGP is located in the flagellar pocket, and contains the linear N-linked polyacetyllactosamine characteristic of the endocytotic machinery of T. brucei. These observations strongly suggest that TgsGP is a T. b. gambiense specific receptor. Since stable expression of this protein in T. b. brucei did not confer resistance to NHS, TgsGP may either need another factor to achieve this purpose or fulfils another function linked to adaptation of the parasite to man.

A VSG expression site-associated gene confers resistance to human serum in Trypanosoma rhodesiense

Infectivity of Trypanosoma brucei rhodesiense to humans is due to its resistance to a lytic factor present in human serum. In the ETat 1 strain this character was associated with antigenic variation, since expression of the ETat 1.10 variant surface glycoprotein was required to generate resistant (R) clones. In addition, in this strain transcription of a gene termed SRA was detected in R clones only. We show that the ETat 1.10 expression site is the one selectively transcribed in R variants. This expression site contains SRA as an expression site-associated gene (ESAG) and is characterized by the deletion of several ESAGs. Transfection of SRA into T.b. brucei was sufficient to confer resistance to human serum, identifying this gene as one of those responsible for T.b. rhodesiense adaptation to humans.

Characterization of the ligand-binding site of the transferrin receptor in Trypanosoma brucei demonstrates a structural relationship with the N-terminal domain of the variant surface glycoprotein

The Trypanosoma brucei transferrin (Tf) receptor is a heterodimer encoded by ESAG7 and ESAG6, two genes contained in the different polycistronic transcription units of the variant surface glycoprotein (VSG) gene. The sequence of ESAG7/6 differs slightly between different units, so that receptors with different affinities for Tf are expressed alternatively following transcriptional switching of VSG expression sites during antigenic variation of the parasite. Based on the sequence homology between pESAG7/6 and the N-terminal domain of VSGs, it can be predicted that the four blocks containing the major sequence differences between pESAG7 and pESAG6 form surface-exposed loops and generate the ligand-binding site. The exchange of a few amino acids in this region between pESAG6s encoded by different VSG units greatly increased the affinity for bovine Tf. Similar changes in other regions were ineffective, while mutations predicted to alter the VSG-like structure abolished the binding. Chimeric proteins containing the N-terminal dimerization domain of VSG and the C-terminal half of either pESAG7 or pESAG6, which contains the ligand-binding domain, can form heterodimers that bind Tf. Taken together, these data provided evidence that the T.brucei Tf receptor is structurally related to the N-terminal domain of the VSG and that the ligand-binding site corresponds to the exposed surface loops of the protein.

Expression of a variant surface glycoprotein of Trypanosoma gambiense in procyclic forms of Trypanosoma brucei shows that the cell type dictates the nature of the glycosylphosphatidylinositol membrane anchor attached to the glycoprotein

Procyclic forms of Trypanosoma brucei have been genetically modified to express the major metacyclic variant surface glycoprotein (VSG variant AnTat 11.17) of Trypanosoma gambiense. The VSG is expressed in an intact membrane-bound form that can be detected over the entire plasma membrane, together with procyclin, and as a series of lower-molecular-mass fragments that are mostly soluble degradation products. The presence of degraded VSG in the cells and the culture medium suggests that VSG is not efficiently processed and/or efficiently folded when expressed in procyclic cells. The level of procyclin expressed on the surface of these cells is slightly reduced, although there is no difference in procyclin mRNA levels. The intact membrane-bound form of the VSG is N-glycosylated with oligomannose structures and contains a glycosylphosphatidylinositol (GPI) membrane anchor that can be biosynthetically labelled with [3H]ethanolamine. The anchor is sensitive to mammalian GPI-specific phospholipase D but, like the anchor of procyclin, it is resistant to the action of bacterial phosphatidylinositol-specific phospholipase C. This pattern of phospholipase sensitivity suggests that the GPI anchor acquired by VSG when expressed in procyclics is acylated on the inositol ring and therefore resembles a procyclic procyclin-type anchor rather than a trypomastigote VSG-type anchor with respect to the lipid structure. The VSG expressed in procyclics was sensitive to the action of a mixture of sialidase, beta-galactosidase and beta-hexosaminidase, suggesting that the VSG GPI anchor also contains a sialylated polylactosamine side-chain modification similar to that described for procyclin. These results indicate that the nature of the protein expressed has little influence on the post-translational modifications performed in the secretory pathway of procyclic trypanosomes.

The primary structure of Trypanosoma (Nannomonas) congolese variant surface glycoproteins

The complete nucleotide sequences were determined for three transcripts each encoding a different variant surface glycoprotein (VSG) of Trypanosoma (Nannomonas) congolense. The nucleotide sequence was determined also for a transcript encoding a fourth VSG, but this was truncated. The data obtained confirm absence of the canonical polyadenylation signal, lack of conserved sequence elements in the 3' untranslated region, and heterogeneity in the spliced-leader acceptor site in the T. congolense VSG transcripts examined. A comparison of the amino acids deduced from the nucleotide sequences of the four VSGs and those of other VSGs published previously reveals a strong conservation of several structural domains, particularly cysteine residues located throughout most of the molecules. The majority of T. congolense VSGs analyzed in this study resemble most the N-terminal cysteine residue domain type B of T. brucei, characterized by a cysteine residue located toward the N-terminal end, a cluster of cysteine residues in the central region, and at least three cysteine residues between positions 250 and 300 of the molecules. One of the VSGs analyzed, ILNat3.3, did not fit into any of the classification schemes proposed for the VSGs so far studied, and thus may represent a different class of these surface molecules. Unlike VSGs of T. brucei, the T. congolense VSGs have no cysteine residues at the carboxy-terminal end. These data now make it possible to predict general primary structural features of T. congolense VSGs.

Targeted disruption of expression site-associated gene-1 in bloodstream-form Trypanosoma brucei

Each variant surface glycoprotein (Vsg) expression site (ES) in bloodstream-form Trypanosoma brucei is a polycistronic transcription unit containing several distinct expression site-associated genes (esag), in addition to a single vsg gene. esag1 genes from different ESs encode a highly polymorphic family of membrane-associated glycoproteins, whose function is unknown. In the hope of producing a phenotype that could indicate a function, we disrupted the esag1 genes in two ESs by targeted insertion of a hygromycin phosphotransferase gene. Our failure to produce an obvious phenotype prompted us to search for other esag1 transcripts. RNA from the mutant trypanosomes hybridized with an esag1-specific oligonucleotide. Cloning and sequencing of mRNA from both mutant and wild-type cells showed that several esag1 family members were expressed, each at a much lower level than the esag1 transcript from the active ES in wild-type trypanosomes. Long-range DNA mapping showed that these additional esag1 genes, some of which contained premature translation-termination codons, most probably originate from chromosomal-internal genes and pseudogenes. We have therefore been unable to determine whether esag1 is an essential gene, or what function it fulfils, or whether any competent Esag1 protein is expressed in the mutant trypanosomes.

DNA rearrangements associated with multiple consecutive directed antigenic switches in Trypanosoma brucei

Changes in variant surface glycoprotein (Vsg) expression allow Trypanosoma brucei to elude the immune response. The expressed vsg is always located at the telomeric end of a polycistronic transcription unit known as an expression site (ES). Although there are many ESs, only one is active at any particular time. The mechanisms regulating ES transcription and switching are unknown. Chromosome rearrangements within or upstream of the ES have been described to occur in occasional switch events, but no changes have been consistently associated with switching. We inserted the drug resistance genes neo and ble, conferring resistance to G418 and phleomycin, respectively, 1 kb downstream of "silent" ES promoters. This demonstrated that short-range transcription could be achieved from a silent ES promoter. From one initial transformant clone, panels of independent consecutive on-off-on switch clones were generated and analyzed. The first activation of the neo-targeted ES was always associated with deletion of the upstream tandem promoter in this ES, but no further rearrangements were detected in consecutive off-on switches of this ES. On the other hand, direct analysis of ES promoters showed that deletions and duplications occurred elsewhere. Activation of a ble-tagged 300-kb chromosome could not be achieved, but phleomycin-resistant clones could be obtained. One such clone arose from recombination between three ESs. Taken together, our experiments suggest that ES switching may occur after a period of chromosomal interactivity that may or may not leave tangible evidence in the form of detectable sequence changes.

Characterization of Trypanosoma brucei gambiense isolates using restriction fragment length polymorphisms in 5 variant surface glycoprotein genes

Fifty-eight Type I Trypanosoma brucei gambiense (G) stocks, including 16 from 3 sleeping sickness foci in Cameroon, were compared by Restriction Fragment Length Polymorphism (RFLP) analysis with 14 T.b. brucei and T.b. rhodesiense stocks from various endemic areas of Africa. Loci examined were for 5 variant surface glycoprotein (VSG) genes: the LiTat 1.3, AnTat 11.17 and 2K genes were present as single copy genes, while the VSG 117 and U2 gene probes hybridised with a family of related genes. The RFLP data were subjected to cluster analysis to produce a dendrogram constructed from similarity coefficients. The LiTat 1.3 and AnTat 11.17 genes are considered to be characteristic of G stocks, and neither gene was found in the non-G stocks; however, the LiTat 1.3 gene was absent from 6 of the 58 G stocks, while the AnTat 11.17 gene was absent from 8. Supplementation of the LiTat 1.3 antigen in the Card Agglutination Test for Trypanosomiasis with the AnTat 11.17 antigen might thus improve performance of the test, particularly in Cameroon. The U2 VSG gene probe gave a characteristic RFLP pattern for G stocks, as did the VSG 117 gene; the latter is an isogene of AnTat 1.8 previously used extensively to characterise G stocks by other workers. The 2K gene was absent in some G stocks, while present in some non-G stocks, and was not therefore useful for characterisation of G stocks. In cluster analysis, the T.b. gambiense stocks formed a large homogeneous group, subdivided into 5 subgroups, with the non-gambiense stocks as a heterogeneous outgroup.

Families of adenylate cyclase genes in Trypanosoma brucei

Four genes for adenylate cyclase have been characterized in Trypanosoma brucei. One of them, esag 4 (for expression site associated gene 4) is present in different VSG (variant surface glycoprotein) gene expression sites and, thus, is only expressed in the bloodstream form of the parasite. The others, termed gresag 4.1, 4.2 and 4.3 (for genes related to esag 4) are expressed in both bloodstream and procyclic forms. In addition, we cloned a esag 4-related gene from T. congolense. Here we characterize the genomic organization of gresag 4.1 and 4.3. While gresag 4.3 is unique, gresag 4.1 exists as a multigenic family of at least nine members located on a 3-Mb chromosome. Six of them are clustered in a region of 300 kb, three copies being tandemly linked. The determination of the nucleotide sequence of a conserved 1.6 kb PstI fragment demonstrated the presence of two separate subgroups in this family. This gene arrangement is present in different isolates of T.b. brucei/rhodesiense/gambiense. Several gresag 4.1 copies are transcribed in both bloodstream and procyclic forms.

Targeting of exogenous DNA into Trypanosoma brucei requires a high degree of homology between donor and target DNA

Integration of exogenous DNA into the trypanosome genome occurs by homologous recombination only. To test whether a high degree of homology between donor and target DNA is required, we have inserted marker genes for drug resistance into the promoter area of variant surface glycoprotein (VSG) gene expression sites of Trypanosoma brucei, using targeting fragments from two expression sites that are 92% identical. We observed integrations into expression sites that are known to be perfectly matched to the donor flanks, and into subsets of uncharacterized expression sites that are specific for each type of targeting fragment, and that could be similar or identical to the donor flanks. This requirement for very high homology was found in both procyclic and bloodstream-form trypanosomes. We speculate that trypanosomes have a mismatch repair system that suppresses recombination between divergent DNA sequences, and we discuss ways in which the trypanosome might circumvent the requirement for perfect DNA homology in the duplicative transposition of a VSG gene into a VSG gene expression site.

Evaluation of variant specific trypanolysis tests for serodiagnosis of human infections with Trypanosoma brucei gambiense

Twelve T.b. gambiense clone populations of distinct Variable Antigen Type (VAT) were combined in immune lysis tests with 340 sera of trypanosome infected patients from 8 different African countries and 267 non trypanosomiasis control sera. The diagnostic specificity of the test was 100%. At a serum dilution of 1:4 the overall test sensitivity with single VATs varied from 39.1 to 98.2% and from 12.1 to 86.8% at 1:32. At a serum dilution of 1:32 some combination tests with 2 VATs still scored above 96%. The VAT recognition patterns were clearly correlated with the geographical origin of the sera, reflecting a diversity in variable antigen repertoires.

Control of gene expression in trypanosomes

Trypanosomes are protozoan agents of major parasitic diseases such as Chagas' disease in South America and sleeping sickness of humans and nagana disease of cattle in Africa. They are transmitted to mammalian hosts by specific insect vectors. Their life cycle consists of a succession of differentiation and growth phases requiring regulated gene expression to adapt to the changing extracellular environment. Typical of such stage-specific expression is that of the major surface antigens of Trypanosoma brucei, procyclin in the procyclic (insect) form and the variant surface glycoprotein (VSG) in the bloodstream (mammalian) form. In trypanosomes, the regulation of gene expression is effected mainly at posttranscriptional levels, since primary transcription of most of the genes occurs in long polycistronic units and is constitutive. The transcripts are processed by transsplicing and polyadenylation under the influence of intergenic polypyrimidine tracts. These events show some developmental regulation. Untranslated sequences of the mRNAs seem to play a prominent role in the stage-specific control of individual gene expression, through a modulation of mRNA abundance. The VSG and procyclin transcription units exhibit particular features that are probably related to the need for a high level of expression. The promoters and RNA polymerase driving the expression of these units resemble those of the ribosomal genes. Their mutually exclusive expression is ensured by controls operating at several levels, including RNA elongation. Antigenic variation in the bloodstream is achieved through DNA rearrangements or alternative activation of the telomeric VSG gene expression sites. Recent discoveries, such as the existence of a novel nucleotide in telomeric DNA and the generation of point mutations in VSG genes, have shed new light on the mechanisms and consequences of antigenic variation.

A monocistronic transcript for a trypanosome variant surface glycoprotein

Many protein-encoding genes of African trypanosomes are transcribed as large polycistronic pre-mRNAs that are processed into individual mRNAs containing a 5' spliced leader and 3' poly(A). The 45- to 60-kb pre-mRNAs encoding some variant surface glycoproteins (VSGs) contain as many as eight unrelated coding regions. Here we identify the promoter for a metacyclic VSG gene that is expressed without duplication in a bloodstream trypanosome clone. This 70-bp promoter is located 2 kb upstream of the telomere-linked VSG gene and directs the synthesis of a monocistronic VSG pre-mRNA lacking the 5' spliced leader. Its sequence only slightly resembles those of other known trypanosome promoters, but it does cross-hybridize with several related sequences elsewhere in the genome. These results suggest that a new class of trypanosome promoters has been found, whose function is to initiate monocistronic transcription of those VSG genes normally expressed during the metacyclic stage.

A monocistronic transcript for a trypanosome variant surface glycoprotein

Many protein-encoding genes of African trypanosomes are transcribed as large polycistronic pre-mRNAs that are processed into individual mRNAs containing a 5' spliced leader and 3' poly(A). The 45- to 60-kb pre-mRNAs encoding some variant surface glycoproteins (VSGs) contain as many as eight unrelated coding regions. Here we identify the promoter for a metacyclic VSG gene that is expressed without duplication in a bloodstream trypanosome clone. This 70-bp promoter is located 2 kb upstream of the telomere-linked VSG gene and directs the synthesis of a monocistronic VSG pre-mRNA lacking the 5' spliced leader. Its sequence only slightly resembles those of other known trypanosome promoters, but it does cross-hybridize with several related sequences elsewhere in the genome. These results suggest that a new class of trypanosome promoters has been found, whose function is to initiate monocistronic transcription of those VSG genes normally expressed during the metacyclic stage.

A new VSG expression site-associated gene (ESAG) in the promoter region of Trypanosoma brucei encodes a protein with 10 potential transmembrane domains

In Trypanosoma brucei bloodstream variants 118 cl 1, 118a and 118b, the actively transcribed VSG gene expression site (ES) is located on a 1.5 Mb chromosome. The promoter region for this polycistronic transcription unit is unusual in that there are two, tandemly located, promoter repeats, each 2.1 kb in size, separated by 13 kb of intervening DNA. As previously shown, at inactivation of this ES, the promoter region was rearranged with the deletion of 15 kb of DNA. This result prompted us to search through the deleted DNA sequences to identify additional genes that might play a role in the inactivation of ESs. In this report, we identify a gene, encoding a putative transmembrane protein, that was deleted at this locus by the rearrangement event. This gene, which we tentatively call expression-site-associated-gene 10 (ESAG10), contains 10 potential transmembrane domains and had been located to T. brucei stock 427-60, ES-containing chromosomes.

The isolation and genetic heterogeneity of Trypanosoma brucei gambiense from north-west Uganda

Fifty-two samples of blood were taken from sleeping sickness patients in north-west Uganda. All samples failed to infect immunosuppressed mice. Ten cryopreserved blood samples were fed to laboratory bred Glossina morsitans morsitans; eight flies developed midgut infections from which procyclic cultures were established in vitro. Isoenzyme electrophoretic analysis of 9 enzymes revealed that 7 of the 8 trypanosome isolates had a combination of enzyme patterns already described for Trypanosoma brucei gambiense. The eighth isolate had a different aspartate aminotransferase polymorphism which placed it in a new zymodeme. Analysis of polymorphisms in genes for 3 variant surface glycoproteins (VSGs) confirmed that the 8 Ugandan trypanosome isolates were T.b.gambiense and revealed further heterogeneity. The VSG 117 gene was present in all the isolates in a pattern of fragments (equivalent to AnTat 1.8) characteristic for T.b.gambiense. For two other VSG genes characteristic of T.b.gambiense, the LiTat 1.3 gene was present in all the isolates, while the AnTat 11.17 gene was present in only 2 of the 8 isolates.

Identification of Trypanosoma brucei gambiense by PCR amplification of variant surface glycoprotein genes

We have developed a sensitive and specific method to identify Trypanosoma brucei gambiense using the polymerase chain reaction (PCR) to amplify the gene encoding variant surface glycoprotein (VSG) Antat 11.17. The test was capable of distinguishing T. b. gambiense from T. b. brucei in most foci of gambian sleeping sickness and gave positive results with previously well-characterised Type I T. b. gambiense stocks from Ivory Coast, Nigeria, Cameroon, Congo, Zaire and Sudan. The test gave negative results with T. b. rhodesiense from Zambia, Kenya and Uganda, virulent or Type II T. b. gambiense from Ivory Coast and T. b. brucei stocks from East and West Africa. The test was modified for colorimetric detection in dot blot format by using nested biotinylated primers in a two-step reaction. Comparison of DNA sequences of VSG genes from T. b. gambiense and other T. brucei ssp. stocks showed a high level of homology, suggesting recent gene flow.