Activation of the genes for variant surface glycoproteins 117 and 118 in Trypanosoma brucei

Synthesis of Galactosylated Glycosylphosphatidylinositol Derivatives from Trypanosoma brucei

Trypanosoma brucei uses variant surface glycoproteins (VSGs) to evade the host immune system and ensure parasitic longevity in animals and humans. VSGs are attached to the cell membrane by complex glycosylphosphatidylinositol anchors (GPI). Distinguishing structural feature of VSG GPIs are multiple α- and β-galactosides attached to the conserved GPI core structure. T. brucei GPIs have been associated with macrophage activation and alleviation of parasitemia during infection, acting as disease onset delaying antigens. Literature reports that link structural modifications in the GPIs to changes in biological activity are contradictory. We have established a synthetic route to prepare structurally overlapping GPI derivatives bearing different T. brucei characteristic structural modifications. The GPI collection will be used to assess the effect of galactosylation and phosphorylation on T. brucei GPI immunomodulatory activity, and to perform an epitope mapping of this complex glycolipid as potential diagnostic marker for Trypanosomiasis. A strategy for the synthesis of a complete α-tetragalactoside using the 2-naphthylmethyl protecting group and for subsequent attachment of GPI fragments to peptides is presented.

Antigenic variation in African trypanosomes: DNA rearrangements program immune evasion

Individual B cells express only one of the many variable-region genes of the VH gene repertoire. Likewise, individual African trypanosomes express only one surface-antigen gene of the large surface-antigen gene repertoire. In both kinds of cells, expression is controlled at the level of transcriptional activation and has been shown to involve rearrangement of genomic DNA. Here, Nina Agabian and her colleagues review recent studies on the molecular mechanisms controlling trypanosome surface-antigen gene expression.

Mosaic VSGs and the scale of Trypanosoma brucei antigenic variation

A main determinant of prolonged Trypanosoma brucei infection and transmission and success of the parasite is the interplay between host acquired immunity and antigenic variation of the parasite variant surface glycoprotein (VSG) coat. About 0.1% of trypanosome divisions produce a switch to a different VSG through differential expression of an archive of hundreds of silent VSG genes and pseudogenes, but the patterns and extent of the trypanosome diversity phenotype, particularly in chronic infection, are unclear. We applied longitudinal VSG cDNA sequencing to estimate variant richness and test whether pseudogenes contribute to antigenic variation. We show that individual growth peaks can contain at least 15 distinct variants, are estimated computationally to comprise many more, and that antigenically distinct 'mosaic' VSGs arise from segmental gene conversion between donor VSG genes or pseudogenes. The potential for trypanosome antigenic variation is probably much greater than VSG archive size; mosaic VSGs are core to antigenic variation and chronic infection.

Antigenic diversity is generated by distinct evolutionary mechanisms in African trypanosome species

Antigenic variation enables pathogens to avoid the host immune response by continual switching of surface proteins. The protozoan blood parasite Trypanosoma brucei causes human African trypanosomiasis ("sleeping sickness") across sub-Saharan Africa and is a model system for antigenic variation, surviving by periodically replacing a monolayer of variant surface glycoproteins (VSG) that covers its cell surface. We compared the genome of Trypanosoma brucei with two closely related parasites Trypanosoma congolense and Trypanosoma vivax, to reveal how the variant antigen repertoire has evolved and how it might affect contemporary antigenic diversity. We reconstruct VSG diversification showing that Trypanosoma congolense uses variant antigens derived from multiple ancestral VSG lineages, whereas in Trypanosoma brucei VSG have recent origins, and ancestral gene lineages have been repeatedly co-opted to novel functions. These historical differences are reflected in fundamental differences between species in the scale and mechanism of recombination. Using phylogenetic incompatibility as a metric for genetic exchange, we show that the frequency of recombination is comparable between Trypanosoma congolense and Trypanosoma brucei but is much lower in Trypanosoma vivax. Furthermore, in showing that the C-terminal domain of Trypanosoma brucei VSG plays a crucial role in facilitating exchange, we reveal substantial species differences in the mechanism of VSG diversification. Our results demonstrate how past VSG evolution indirectly determines the ability of contemporary parasites to generate novel variant antigens through recombination and suggest that the current model for antigenic variation in Trypanosoma brucei is only one means by which these parasites maintain chronic infections.

Analysis of the VSG gene silent archive in Trypanosoma brucei reveals that mosaic gene expression is prominent in antigenic variation and is favored by archive substructure

Trypanosoma brucei evades host acquired immunity through differential activation of its large archive of silent variant surface glycoprotein (VSG) genes, most of which are pseudogenes in subtelomeric arrays. We have analyzed 940 VSGs, representing one half to two thirds of the arrays. Sequence types A and B of the VSG N-terminal domains were confirmed, while type C was found to be a constituent of type A. Two new C-terminal domain types were found. Nearly all combinations of domain types occurred, with some bias to particular combinations. One-third of encoded N-terminal domains, but only 13% of C-terminal domains, are intact, indicating a particular need for silent VSGs to gain a functional C-terminal domain to be expressed. About 60% of VSGs are unique, the rest occurring in subfamilies of two to four close homologs (>50%-52% peptide identity). We found a subset of VSG-related genes, differing from VSGs in genomic environment and expression patterns, and predict they have distinct function. Almost all (92%) full-length array VSGs have the partially conserved flanks associated with the duplication mechanism that activates silent genes, and these sequences have also contributed to archive evolution, mediating most of the conversions of segments, containing >/=1 VSG, within and between arrays. During infection, intact array genes became activated by duplication after two weeks, and mosaic VSGs assembled from pseudogenes became expressed by week three and predominated by week four. The small subfamily structure of the archive appears to be fundamental in providing the interacting donors for mosaic formation.

VSG switching in Trypanosoma brucei: antigenic variation analysed using RNAi in the absence of immune selection

Trypanosoma brucei relies on antigenic variation of its variant surface glycoprotein (VSG) coat for survival. We show that VSG switching can be efficiently studied in vitro using VSG RNAi in place of an immune system to select for switch variants. Contrary to models predicting an instant switch after inhibition of VSG synthesis, switching was not induced by VSG RNAi and occurred at a rate of 10(-4) per division. We find a highly reproducible hierarchy of VSG activation, which appears to be capable of resetting, whereby more than half of the switch events over 12 experiments were to one of two VSGs. We characterized switched clones according to switch mechanism using marker genes in the active VSG expression site (ES). Transcriptional switches between ESs were the preferred switching mechanism, whereby at least 10 of the 17 ESs identified in T. brucei 427 can be functionally active in vitro. We could specifically select for switches mediated by DNA rearrangements by inducing VSG RNAi in the presence of drug selection for the active ES. Most of the preferentially activated VSGs could be activated by multiple mechanisms. This VSG RNAi-based procedure provides a rapid and powerful means for analysing VSG switching in African trypanosomes entirely in vitro.

What the genome sequence is revealing about trypanosome antigenic variation

African trypanosomes evade humoral immunity through antigenic variation, whereby they switch expression of the gene encoding their VSG (variant surface glycoprotein) coat. Switching proceeds by duplication of silent VSG genes into a transcriptionally active locus. The genome project has revealed that most of the silent archive consists of hundreds of subtelomeric VSG tandem arrays, and that most of these are not functional genes. Precedent suggests that they can contribute combinatorially to the formation of expressed, functional genes through segmental gene conversion. These findings from the genome project have major implications for evolution of the VSG archive and for transmission of the parasite in the field.

Two pathways of homologous recombination in Trypanosoma brucei

African trypanosomes are unicellular parasites that use DNA recombination to evade the mammalian immune response. They do this in a process called antigenic variation, in which the parasites periodically switch the expression of VSG genes that encode distinct Variant Surface Glycoprotein coats. Recombination is used to move new VSG genes into specialised bloodstream VSG transcription sites. Genetic and molecular evidence has suggested that antigenic variation uses homologous recombination, but the detailed reaction pathways are not understood. In this study, we examine the recombination pathways used by trypanosomes to integrate transformed DNA into their genome, and show that they possess at least two pathways of homologous recombination. The primary mechanism is dependent upon RAD51, but a subsidiary pathway exists that is RAD51-independent. Both pathways contribute to antigenic variation. We show that the RAD51-independent pathway is capable of recombining DNA substrates with very short lengths of sequence homology and in some cases aberrant recombination reactions can be detected using such microhomologies.

Predominance of duplicative VSG gene conversion in antigenic variation in African trypanosomes

A number of mechanisms have been described by which African trypanosomes undergo the genetic switches that differentially activate their variant surface glycoprotein genes (VSGs) and bring about antigenic variation. These mechanisms have been observed mainly in trypanosome lines adapted, by rapid syringe passaging, to laboratory conditions. Such "monomorphic" lines, which routinely yield only the proliferative bloodstream form and do not develop through their life cycle, have VSG switch rates up to 4 or 5 orders of magnitude lower than those of nonadapted lines. We have proposed that nonadapted, or pleomorphic, trypanosomes normally have an active VSG switch mechanism, involving gene duplication, that is depressed, or from which a component is absent, in monomorphic lines. We have characterized 88 trypanosome clones from the first two relapse peaks of a single rabbit infection with pleomorphic trypanosomes and shown that they represent 11 different variable antigen types (VATs). The pattern of appearance in the first relapse peak was generally reproducible in three more rabbit infections. Nine of these VATs had activated VSGs by gene duplication, the tenth possibly also had done so, and only one had activated a VSG by the transcriptional switch mechanism that predominates in monomorphic lines. At least 10 of the donor genes have telomeric silent copies, and many reside on minichromosomes. It appears that trypanosome antigenic variation is dominated by one, relatively highly active, mechanism rather than by the plethora of pathways described before.

Mutational analysis of the C-terminal signal peptide of bovine liver 5'-nucleotidase for GPI anchoring: a study on the significance of the hydrophilic spacer region

Bovine liver 5'-nucleotidase is a GPI-anchored protein whose Ser523 attaches to GPI as the omega-site. For GPI-modification, pro-protein of the enzyme possesses a signal peptide at the C-terminus, comprising a hydrophilic spacer sequence of 8 amino acid residues and the following hydrophobic region of 17 amino acid residues. The C-terminal signal peptide is replaced by GPI on a luminal leaflet of endoplasmic reticulum. To characterize the C-terminal signal peptide for GPI modification, we constructed a series of deletion and elongation mutant genes, altering length of the hydrophilic spacer sequence by site-directed mutagenesis. Systematic deletion and Ala insertion of the sequence showed that the sequence of 6-14 residues were compatible for GPI modification. For GPI transfer to the pro-protein, the optimum length of spacer sequence would be 8, being consistent with natural selection. The spacer sequence may play a role for leading the omega-residue correctly to the active site of putative GPI transamidase. The elongation of the spacer is more permissible than deletion. Nevertheless, the length of the spacer sequence may influence efficiency of GPI modification by its positive or negative control.

The relative significance of mechanisms of antigenic variation in African trypanosomes

The large number of genes involved in antigenic variation in African trypanosomes has been the focus of a wide literature that describes an almost bewildering array of mechanisms for their differential activation. To the outsider searching for an underlying strategy for antigenic variation, this can appear as a rather disordered and confusing picture. Here, David Barry argues that an understanding of which mechanisms are significant, which ones are primarily inconsequential and which ones perhaps even arise from overdependence on laboratory models, might be achieved by turning attention to trypanosomes that have not undergone adaptation in laboratory conditions. Application of such an approach has led to a proposal for a main mechanism for antigenic variation.

Gene conversions mediating antigenic variation in Trypanosoma brucei can occur in variant surface glycoprotein expression sites lacking 70-base-pair repeat sequences

African trypanosomes undergo antigenic variation of their variant surface glycoprotein (VSG) coat to avoid immune system-mediated killing by their mammalian host. An important mechanism for switching the expressed VSG gene is the duplicative transposition of a silent VSG gene into one of the telomeric VSG expression sites of the trypanosome, resulting in the replacement of the previously expressed VSG gene. This process appears to be a gene conversion reaction, and it has been postulated that sequences within the expression site may act to initiate and direct the reaction. All bloodstream form expression sites contain huge arrays (many kilobase pairs) of 70-bp repeat sequences that act as the 5' boundary of gene conversion reactions involving most silent VSG genes. For this reason, the 70-bp repeats seemed a likely candidate to be involved in the initiation of switching. Here, we show that deletion of the 70-bp repeats from the active expression site does not affect duplicative transposition of VSG genes from silent expression sites. We conclude that the 70-bp repeats do not appear to function as indispensable initiation sites for duplicative transposition and are unlikely to be the recognition sequence for a sequence-specific enzyme which initiates recombination-based VSG switching.

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.

Residues in Torpedo californica acetylcholinesterase necessary for processing to a glycosyl phosphatidylinositol-anchored form

Acetylcholinesterase from Torpedo californica (TcAChE) can be found as a glycosyl phosphatidylinositol (GPI)-anchored, membrane associated form. The C-terminal amino-acid sequence of the precursor protein resembles the signal peptide sequence found in proteins and enzymes destined for GPI-modification. Characteristics of such a signal peptide are a relatively polar stretch of amino acids, separating a cleavage- and modification-site (omega-site) residue from a hydrophobic C-terminus. We have introduced mutations, both at putative omega-sites and in the hydrophobic region, and analysed their effects on GPI-anchoring of TcAChE. Our results show that substitution of all three Ser residues in the region Ser542-Ser544 prevents GPI-modification and membrane anchoring. Individual substitution of each of these residues resulted in no or only a minor effect on the modification. We therefore conclude that more than one residue within this sequence can be utilised as the omega-site. Our analyses of double substitutions indicated that Ser543 and Ser544 are the preferred residues for GPI-modification. Moreover, the hydrophobic region is shown to be essential for GPI-anchoring of TcAChE.

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.

Hierarchies of RNA-processing signals in a trypanosome surface antigen mRNA precursor

Nearly all trypanosome mRNAs are synthesized as polycistronic precursors, from which mature mRNAs are excised by trans splicing and polyadenylation. Polyadenylation of a procyclic acidic repetitive protein (PARP, or procyclin) transcript was studied by transient transfection of constructs bearing a chloramphenicol acetyltransferase gene linked to the PARP intergenic region. Polyadenylation usually occurred at A residues, about 100 bases upstream of a trans-splicing acceptor signal. The wild-type polyadenylation site has a cryptic trans-splicing signal about 100 bp downstream: deletion or inversion of this signal results in polyadenylation at multiple sites, upstream of other cryptic trans-splicing signals. The PARP mRNA precursor appears to contain a hierarchy of possible processing signals, the function of cryptic ones being revealed only when the dominant ones are deleted or moved. Correct polyadenylation can be restored by addition of trans-splicing signals from other loci. The results indicate that polyadenylation is coupled to downstream trans splicing but that the products of the trans-splicing reaction are not necessarily functional mRNAs.

Hierarchies of RNA-processing signals in a trypanosome surface antigen mRNA precursor

Nearly all trypanosome mRNAs are synthesized as polycistronic precursors, from which mature mRNAs are excised by trans splicing and polyadenylation. Polyadenylation of a procyclic acidic repetitive protein (PARP, or procyclin) transcript was studied by transient transfection of constructs bearing a chloramphenicol acetyltransferase gene linked to the PARP intergenic region. Polyadenylation usually occurred at A residues, about 100 bases upstream of a trans-splicing acceptor signal. The wild-type polyadenylation site has a cryptic trans-splicing signal about 100 bp downstream: deletion or inversion of this signal results in polyadenylation at multiple sites, upstream of other cryptic trans-splicing signals. The PARP mRNA precursor appears to contain a hierarchy of possible processing signals, the function of cryptic ones being revealed only when the dominant ones are deleted or moved. Correct polyadenylation can be restored by addition of trans-splicing signals from other loci. The results indicate that polyadenylation is coupled to downstream trans splicing but that the products of the trans-splicing reaction are not necessarily functional mRNAs.

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.

Early expression of a Trypanosoma brucei VSG gene duplicated from an incomplete basic copy

Intrachromosomal variant surface glycoprotein (VSG) genes in Trypanosoma brucei are expressed by a mechanism involving gene conversion. The 3' boundary of gene conversion is usually within the last 130 bp of the VSG gene, a region of partially conserved sequences. We report here the loss of the predominant telomeric A VSG gene in the cloned variant antigenic type (VAT) 5A3, leaving only an intrachromosomal A VSG gene (the A-B gene). The nucleotide sequence of the A-B VSG gene reveals that it lacks the normal VSG 3' sequence. Surprisingly, we find cells expressing this A-B VSG gene in relapse populations arising from VAT 5A3. Since the A VSG mRNAs from these cells have a normal 3' sequence, the incomplete A-B VSG gene must be expressed via a partial gene conversion that supplies the functional 3' end. Although the A-B VSG gene is no longer predominant like the telomeric A VSG gene, it is still expressed more frequently than other intrachromosomal VSG genes, suggesting that factors other than a telomeric location determine whether a VSG gene is expressed early in a serodeme.

A proposed mechanism for promoter-associated DNA rearrangement events at a variant surface glycoprotein gene expression site

The expressed variant cell surface glycoprotein (VSG) gene of the protozoan parasite Trypanosoma brucei is invariably found at one of several telomeric VSG gene expression sites (ESs). The active ES in variant 118 clone 1 is found on a 1.5-Mb chromosome, and the promoter region is located more than 45 kb upstream of the VSG gene. We had previously shown that DNA rearrangement events occurred in the promoter region, specifically at inactivation of this ES (K. M. Gottesdiener, H.-M. Chung, S. L. Brown, M. G.-S. Lee, and L. H. T. Van der Ploeg, Mol. Cell. Biol. 11:2467-2477, 1991). In this report, we describe the cloning of the entire 17-kb promoter region, which revealed the presence of two identical 2.15-kb tandem promoter repeats separated by 13 kb of DNA. The two virtually identical promoter repeats both function efficiently in directing transcription in transient transfection assays in insect-form trypanosomes. We characterized the DNA rearrangement events that occur at ES inactivation, and by studying both of the reciprocal products of this recombination event, we infer that these result from direct (promoter) repeat recombination, formation of heteroduplex DNA, and a reciprocal exchange event that releases a circular DNA as a side product of the reaction. The finding of DNA recombinational events in a region of the VSG gene ES that encodes the promoter(s), and their relatively frequent occurrence at ES inactivation, suggests a possible role in ES control.

A proposed mechanism for promoter-associated DNA rearrangement events at a variant surface glycoprotein gene expression site

The expressed variant cell surface glycoprotein (VSG) gene of the protozoan parasite Trypanosoma brucei is invariably found at one of several telomeric VSG gene expression sites (ESs). The active ES in variant 118 clone 1 is found on a 1.5-Mb chromosome, and the promoter region is located more than 45 kb upstream of the VSG gene. We had previously shown that DNA rearrangement events occurred in the promoter region, specifically at inactivation of this ES (K. M. Gottesdiener, H.-M. Chung, S. L. Brown, M. G.-S. Lee, and L. H. T. Van der Ploeg, Mol. Cell. Biol. 11:2467-2477, 1991). In this report, we describe the cloning of the entire 17-kb promoter region, which revealed the presence of two identical 2.15-kb tandem promoter repeats separated by 13 kb of DNA. The two virtually identical promoter repeats both function efficiently in directing transcription in transient transfection assays in insect-form trypanosomes. We characterized the DNA rearrangement events that occur at ES inactivation, and by studying both of the reciprocal products of this recombination event, we infer that these result from direct (promoter) repeat recombination, formation of heteroduplex DNA, and a reciprocal exchange event that releases a circular DNA as a side product of the reaction. The finding of DNA recombinational events in a region of the VSG gene ES that encodes the promoter(s), and their relatively frequent occurrence at ES inactivation, suggests a possible role in ES control.

Surface epitope variation via mosaic gene formation is potential key to long-term survival of Trypanosoma brucei

Trypanosoma brucei evades the immune response of its mammalian host by antigenic variation in the major surface antigen (the variable surface glycoprotein or VSG). We examined the generation of diversity in 4 in vivo-derived antigenically related clones of T. brucei by sequencing VSG cDNA from each of the 4 clones and all 5 related genomic copies in the WaTat 1.1 progenitor organism. Each expressed VSG gene was a different mosaic of basic copy genes; 3 were complex mosaics consisting of multiple fragments from at least 3 basic copy genes. All 4 basic copy genes were involved in mosaic gene formation even though at least 2 were pseudogenes. Point mutations were a minor component to VSG variability. We conclude that, in vivo, expression of mosaic VSG genes amplifies the effective surface antigen repertoire of T brucei. We propose that this additional source of antigenic variation is crucial to long term survival of the parasite in its mammalian host, and may be the primary function of VSG multigene families in trypanosomes.

Trypanosoma brucei brucei: antigenic stability of its LDL receptor and immunological cross-reactivity with the LDL receptor of the mammalian host

The rapid growth of Trypanosoma brucei brucei in the blood and tissue fluids of vertebrates requires the receptor-mediated endocytosis of LDL from the host (Coppens et al. 1987; Gillett and Owen 1987) and is slowed by a monospecific rabbit antiserum against the purified LDL receptor of the parasite. We have used this antiserum in combination with several well-characterized antigenic variants (originating from stock 427: MITat 1.1a, 1.3a, 1.4a, 1.5a, 1.5d, 1.8b) to examine whether the LDL receptor of T. b. brucei is a stable surface antigen, common to all parasite variants despite antigenic variation of the major surface glycoprotein, and whether it is immunologically distinct from the LDL receptor of the host. At low concentrations, binding at 4 degrees C of rat LDL to several variants of T. b. brucei and to isolated rat hepatocytes was inhibited to a similar extent by the antiserum. In double immunodiffusion, a single precipitation line was observed, showing continuity between the extracts of all variants as well as between that of trypanosomes and of mammalian tissues. In Western blots of trypanosome extracts, the LDL receptor was strongly labeled as a single band of Mr 145,000, whereas with a rat liver extract, a single band of similar electrophoretic mobility was weakly labeled at a high concentration of the antiserum. In conclusion, the LDL receptor occurred in all variants of T. b. brucei, was a stable surface antigen despite variation of the major surface glycoprotein, and displayed biochemical and immunological similarities with the LDL receptor of the rat host.

Molecular karyotype analysis in Leishmania

The advent of pulsed field electrophoresis has allowed a direct approach to the karyotype of Leishmania. The molecular karyotype thus obtained is a stable characteristic of a given strain, although minor modifications may occur during in vitro maintenance. Between 20 and 28 chromosomal bands can be resolved depending on the strain, ranging in size from approximately 250 to 2600 kb. The technique has revealed a striking degree of polymorphism in the size and number of the chromosomal bands between different strains, and this seems independent of the category (species, zymodeme, population) to which the strains belong. It appears that only certain strains originating from the same geographic area may share extensive similarities. This polymorphism can largely be accounted for by chromosome size variations, which can involve up to 25% of the chromosome length. As a result, homologous chromosomes can exist in versions of markedly different size within the same strain. When this occurs with several different chromosomes, the interpretation of PFE patterns appears difficult without prior identification of the size-variable chromosomes and of the chromosome homologies. DNA deletions and amplifications have been shown to account for some of these size modifications, but other mechanisms are probably involved; nevertheless, interchromosomal exchange does not seem to play a major role in these polymorphisms. These chromosomal rearrangements, yet in an early stage of characterization, exhibit two relevant features: they seem (1) to affect essentially the subtelomeric regions and (2) to occur in a recurrent nonrandom manner. Chromosomal rearrangements sharing the same characteristics have been identified in yeast and other protozoa such as Trypanosoma and Plasmodium. The significance of this hypervariability for the biology of the parasite remains unknown, but it can be expected that such mechanisms have been maintained for some purpose; genes specifically located near chromosome ends might benefit from rapid sequence change, alternating activation, or polymorphism of expression. The chromosomal plasticity could represent a general mode of mutation in these parasites, in parallel with genetic exchange which may be uncommon in nature. The molecular characterization of these rearrangements, the identification of each chromosome with the help of physical restriction maps and linkage maps, and the collation of such data on a number of strains and species should allow a significant progress in the understanding of the genetics of Leishmania, in particular as regards ploidy, generation of phenotypic diversity, and genome evolution. Finally, like other models, this is susceptible to improve our knowledge of DNA-DNA interactions and of the chromosome functional structure and dynamics.

Characterization of VSG gene expression site promoters and promoter-associated DNA rearrangement events

The expressed variant cell surface glycoprotein (VSG) gene of Trypanosoma brucei is located at the 3' end of a large, telomeric, polycistronic transcription unit or expression site. We show that the region 45 kb upstream of the VSG gene, in the expression site on a 1.5-Mb chromosome, contains at least two promoters that are arranged in tandem, directing the transcription of the expression site. DNA rearrangement events occur specifically, at inactivation of the expression site, and these events delete the most upstream transcribed region and replace it with a large array of simple-sequence DNA, leaving the downstream promoter intact. Because of the placement of simple-sequence DNA, the remaining downstream promoter now becomes structurally identical to previously described VSG promoters. The downstream promoter is repetitive in the genome, since it is present at several different expression sites. Restriction fragment length polymorphism mapping allows grouping of the expression sites into two families, those with and those without an upstream transcription unit, and the DNA rearrangement events convert the expression sites from one type to the other. Deletion of the upstream transcription unit also leads to the loss of several steady-state RNAs. The findings may indicate a role for promoter-associated DNA rearrangement events, and/or interactions between tandemly arranged promoters, in expression site transcriptional control.

Characterization of VSG gene expression site promoters and promoter-associated DNA rearrangement events

The expressed variant cell surface glycoprotein (VSG) gene of Trypanosoma brucei is located at the 3' end of a large, telomeric, polycistronic transcription unit or expression site. We show that the region 45 kb upstream of the VSG gene, in the expression site on a 1.5-Mb chromosome, contains at least two promoters that are arranged in tandem, directing the transcription of the expression site. DNA rearrangement events occur specifically, at inactivation of the expression site, and these events delete the most upstream transcribed region and replace it with a large array of simple-sequence DNA, leaving the downstream promoter intact. Because of the placement of simple-sequence DNA, the remaining downstream promoter now becomes structurally identical to previously described VSG promoters. The downstream promoter is repetitive in the genome, since it is present at several different expression sites. Restriction fragment length polymorphism mapping allows grouping of the expression sites into two families, those with and those without an upstream transcription unit, and the DNA rearrangement events convert the expression sites from one type to the other. Deletion of the upstream transcription unit also leads to the loss of several steady-state RNAs. The findings may indicate a role for promoter-associated DNA rearrangement events, and/or interactions between tandemly arranged promoters, in expression site transcriptional control.

A novel DNA nucleotide in Trypanosoma brucei only present in the mammalian phase of the life-cycle

The existence of an unusual form of DNA modification in the bloodstream form of the African trypanosome Trypanosoma brucei has been inferred from partial resistance to cleavage of nuclear DNA with PstI and PvuII (Bernards et al, 1984; Pays et al, 1984). This putative modification is correlated with the shut-off of telomeric Variant-specific Surface Glycoprotein (VSG) gene expression sites (ESs). The modification only affects inactive VSG genes with a telomeric location, and it is absent in procyclic (insect form) trypanosomes in which no VSG is made at all. Previous attempts to detect unusual nucleosides in T.brucei DNA were unsuccessful, but we now report the detection of two unusual nucleotides, called pdJ and pdV, in T.brucei DNA, using the 32P-postlabeling technique. Nucleotide pdV was present in both bloodstream form and procyclic T.brucei DNA and co-migrated in two different two-dimensional thin layer chromatography (2D-TLC) systems with hydroxymethyldeoxyuridine 5'-monophosphate (pHOMedU). In contrast, nucleotide pdJ was exclusively present in bloodstream form trypanosomal DNA. Levels of pdJ were higher in DNA enriched for telomeric sequences than in total genomic DNA and pdJ was also detected in other Kinetoplastida species exhibiting antigenic variation. Postlabeling and 2D-TLC analyses showed base J to be different from the known eukaryotic unusual DNA bases 5-methylcytosine, N6-methyladenine and hydroxymethyluracil, and also from (glucosylated) hydroxymethylcytosine, uracil, alpha-putrescinylthymine, 5-dihydroxypentyluracil and N6-carbamoylmethyladenine. We conclude that pdJ is a novel eukaryotic DNA nucleotide and that it is probably responsible for the partial resistance to cleavage by PvuII and PstI of inactive telomeric VSG genes. It may therefore be involved in the regulation of ES activity in bloodstream form trypanosomes.

Antigenic variation in Trypanosoma brucei: a telomeric expression site for variant-specific surface glycoprotein genes with novel features

African trypanosomes evade the immune response of their host by periodically changing their variant surface glycoprotein (VSG) coat. Each coat is encoded by a separate VSG gene. Expressed genes are in a telomeric expression site (ES) and there are several sites in each trypanosome. To study the transcription control of VSG genes in Trypanosoma brucei we have analyzed an ES, called the dominant ES (DES), that readily switches off and on. The promoter area of the DES is very similar to that of the 221 ES (Zomerdijk et al., 1990). It can be switched off and on in vivo without detectable DNA alterations in the vicinity of the transcription start and it can drive high transient expression of a reporter gene in transfection experiments. However, there are also two major differences between the DES and the 221 ES. First, one version of the DES contains an additional upstream transcription unit overlapping the VSG gene ES promoter. The presence of this upstram transcription is dispensable, however, for the VSG gene ES promoter is active, even if transcription through this start from the upstream promoter is blocked using UV light. Moreover, a second version of the DES present in another trypanosome variant does not produce these upstream transcripts. Secondly, we find that the inactivation of DES transcription in one trypanosome variant is accompanied by DNA alterations in the DES upstream (greater than 2 kb) of the transcription start; reactivation of DES transcription is accompanied by another alteration far upstream. Although we cannot exclude that these DNA rearrangements are incidental, our results raise the possibility that the activity of ES promoters is negatively controlled in cis by far upstream sequences not included in transfection constructs and that alterations in these sequences may lead to (in)activation of the promoter.

Distinct, developmental stage-specific activation mechanisms of trypanosome VSG genes

The metacyclic form of African trypanosomes is the first to express genes for the Variant Surface Glycoprotein (VSG) and it uses an unusually predictable subset of the VSG gene repertoire. We have developed a model system for the analysis of metacyclic VSG (M-VSG) gene expression and have used this to demonstrate that, for two M-VSG genes, different modes of expression operate in the insect and mammalian phases of the life-cycle. In metacyclic-derived clones, these genes are expressed in situ, whereas they are routinely activated by duplication in bloodstream trypanosomes. The expression loci for both M-VSG genes studied are structurally simple and we present a model, based on this, for the maintenance of a separate M-VSG repertoire and expression system.

The Trypanosoma brucei protein phosphatase gene: polycistronic transcription with the RNA polymerase II largest subunit gene

We have previously described the trypanosomal gene encoding the largest subunit of RNA polymerase II (RNAP II) and found that two almost identical genes are encoded within the Trypanosoma brucei genome. Here we show by Southern analyses that the 5' breakpoint between both loci is located approximately 7.5 kb upstream of the RNAP II genes. Northern analyses revealed that the 5' duplicated segment contains at least four other genes, which are transcribed in both bloodstream and procyclic trypanosomes. The gene located immediately upstream of the RNAP II gene in both loci was characterized by sequence analyses. The deduced amino acid sequences show a high degree of similarity to the catalytic subunit of protein phosphatase class 1 (PP1) genes. S1 mapping provided strong evidence in support of the fact that the PP1 and RNAP II genes belong to a single transcription unit.

Trypanosome variable surface glycoproteins: composite genes and order of expression

Combinatorial processes increase the diversity of variable surface glycoproteins (VSGs) expressed by Trypanosoma equiperdum. We show here that a single telomeric pseudogene provides the 3' portion of three distinct T. equiperdum VSG genes by recombination with different 5' donor pseudogenes. Regions of sequence homology among the pseudogenes determine the sites of recombination in the formation of the expressed copies. This suggests that the recombination between any given basic copy (BC) and the expression-linked copy (ELC) depends on their sharing homology. We present evidence that this is the case and propose that such rules account for the order of expression of the VSGs. These results demonstrate how homologous recombination can generate an ordered sequence of gene expression.

Structure of a telomeric expression site for variant specific surface antigens in Trypanosoma brucei

We have studied the organization of the expression site, in which most chromosome-internal variant-specific surface glycoprotein (VSG) genes of Trypanosoma brucei strain 427 are expressed (the dominant expression site) and compared it to the previously characterized VSG 221 expression site. With the exception of a 500 bp segment and a VSG pseudogene, which are absent from the dominant expression site, overall all major sequence elements of the two sites are organized similarly, as judged from their relative mapping positions by UV inactivation of transcription. Transcription is insensitive to 1 mg alpha-amanitin per ml, a characteristic property of VSG gene expression sites analyzed thus far. The sequence elements of the dominant expression site include at least one other expressed gene of unknown function and homologues of at least two other open reading frames. The large internal duplication of the 60-kb 221 expression site appear to be missing from the dominant site, resulting in a shorter, 40-kb transcription unit. As judged from its relative sensitivity to UV inactivation of transcription, a subsidiary promoter, identified by other methods in the dominant expression site appears fully dependent for its activity on the promoter located 40 kb upstream of the VSG gene. We conclude that all VSG gene expression sites may be similarly organized as large polygenic transcription units.

Improved separation of chromosome-sized DNA from Trypanosoma brucei, stock 427-60

Separation of chromosome-sized DNA from the parasitic protozoan Trypanosoma brucei had previously resulted in the fractionation of DNA molecules that ranged in size from 50 kb up to roughly 1.5 Mb. The number of larger chromosomes and their size, accounting for 80% of the DNA of T. brucei remained unclear. We have now size separated these larger DNA molecules by pulsed field gel electrophoresis (PFG) and resolve a total of 20 bands, accounting for roughly 120 chromosomes, ranging in size from 50 kb up to the size of the largest, 5.7 Mb chromosome of Schizosaccharomyces pombe. Three different VSG gene expression sites were located to chromosomes of 430 kb, 1.5 Mb and 3 Mb, respectively. We have not been able to identify additional, previously cryptic DNA rearrangements, that could explain the activation or inactivation of the expression sites.

Trypanosoma brucei: frequent loss of a telomeric variant surface glycoprotein gene

We have observed the loss of an inactive telomeric variant surface glycoprotein (VSG) gene that is located on a minichromosome in Trypanosoma brucei. If this is due to gene conversion, it is the third "silent" gene conversion (i.e., one that does not produce an antigenic switch) detected in 19 antigenic switches of the IsTaR 1 serodeme. This is surprisingly frequent since the immune response cannot select against the inactive gene. We estimate that 10(-1) to 10(-3) telomeric VSG gene conversions occur per generation, which is at least 100 times more frequent than antigenic switching. Since all three "silent" gene conversions involved an IsTat 5 VSG gene, the frequency may vary among telomeric VSG genes. However, the high gene conversion frequency for the 5 VSG gene does not ensure a higher antigenic switch frequency than other telomeric VSG genes for which we have probes. These results suggest that gene conversion rapidly alters the repertoire of telomeric VSG genes, possibly including those on minichromosomes, producing a continual variation in the VSG genes that are more likely to be expressed.

Post-transcriptional control of the differential expression of phosphoglycerate kinase genes in Trypanosoma brucei

The genes for the cytosolic and glycosomal phosphoglycerate kinases (PGK) of Trypanosoma brucei are found in a compact tandem array together with a third PGK-related gene, expressed at low level. Expression of the two PGK genes is differentially regulated in the life cycle of T. brucei: the glycosomal PGK and its mRNA are abundant in the mammalian stage of the cycle but not in the insect stage, whereas the reverse is found for the cytosolic PGK and its mRNA. Nevertheless, our experiments indicate that the mRNAs for both isoenzymes are derived from a common precursor. Nuclease protection experiments using fragments cloned into single-stranded DNA vectors show the presence of low abundance RNA species running through one gene into the next. Indeed minor RNA species larger than the mature mRNAs are visible in overexposed RNA blots. Analysis of nascent RNA in a nuclear run-on assay indicates that the entire PGK gene array is transcribed at an equal rate throughout in both life cycle stages. We conclude that the PGK genes are part of one large multicistronic transcription unit, which is processed to yield the individual mRNAs with concomitant addition of the 5' 35-nucleotide mini-exon sequence, characteristic of all trypanosome mRNAs. It follows that the steady-state levels of the PGK mRNAs are controlled post-transcriptionally.

Stable variant-specific transcripts of the variant cell surface glycoprotein gene 1.8 expression site in Trypanosoma brucei

The structure and transcriptional regulation of the 1.8 variant cell surface glycoprotein (VSG) gene expression site located on a 430-kilobase (kb) chromosome was examined in a 430-kb-chromosome-specific library. Using 32P-labeled nascent transcripts generated by nuclear run-on, we selected recombinant clones derived from the 430-kb chromosome which were coordinately activated with the 1.8 VSG gene. The results show that a repetitive region with a minimum size of 27 kb is coordinately activated with the 1.8 VSG gene. As with the 1.8 VSG gene, transcription is by RNA polymerases that are insensitive to the drug alpha-amanitin at concentrations up to 1 mg/ml. Transcription results in the generation of several stable variant-specific mRNAs. These mRNAs most likely belong to a family of repetitive expression-site-associated genes.

Cloning and transcriptional analysis of a variant surface glycoprotein gene expression site in Trypanosoma brucei

The variant surface glycoprotein (VSG) gene expression site in Trypanosoma brucei variant 117a has been mapped to a point about 40 kb upstream from the VSG gene. Sequences upstream from the previously identified [Cully, D.F. et al. (1985) Cell 42, 173-182] expression site associated gene (ESAG-I) have been cloned and a stable 1.3 kb transcript has been localized immediately 5' to ESAG-I. This transcript is in the same orientation and approximately as abundant as the ESAG-I message. A highly conserved region, of which at least 15 copies are present in the genome, has been identified further upstream. A stable transcript corresponding to this region was not detected in variant 117a, but a 1.7 kb transcript was detected in variant 221a. In isolated nuclei, representative sequences from the 117a, expression site were transcribed unidirectionally at similar rates, and transcription was insensitive to alpha-amanitin.

Organization and interviral homologies of the coat protein gene of white clover mosaic virus

The sequence of 1612 nucleotides of the 3'-terminal region of white clover mosaic virus (WCIMV) has been determined from cDNA clones. The viral sense RNA contains four open reading frames of Mr 20,684, Mr 7219, Mr 12,989, and at least Mr 17,000. The latter begins 5' to the sequence determined. The amino acid sequence of the open reading frame encoding the 20,684 polypeptide shows marked homology to the coat proteins of three other potexviruses. The putative coat protein gene was subcloned in a T7 transcription plasmid and RNAs produced by in vitro transcription were translated in the rabbit reticulocyte lysate system. The polypeptide products comigrated on SDS-polyacrylamide gels with one of those synthesized by the in vitro translation of viral RNA, and were immunoprecipitable with antiserum raised against WCIMV, confirming the location of the coat protein gene.

Stable variant-specific transcripts of the variant cell surface glycoprotein gene 1.8 expression site in Trypanosoma brucei

The structure and transcriptional regulation of the 1.8 variant cell surface glycoprotein (VSG) gene expression site located on a 430-kilobase (kb) chromosome was examined in a 430-kb-chromosome-specific library. Using 32P-labeled nascent transcripts generated by nuclear run-on, we selected recombinant clones derived from the 430-kb chromosome which were coordinately activated with the 1.8 VSG gene. The results show that a repetitive region with a minimum size of 27 kb is coordinately activated with the 1.8 VSG gene. As with the 1.8 VSG gene, transcription is by RNA polymerases that are insensitive to the drug alpha-amanitin at concentrations up to 1 mg/ml. Transcription results in the generation of several stable variant-specific mRNAs. These mRNAs most likely belong to a family of repetitive expression-site-associated genes.

The anatomy and transcription of a telomeric expression site for variant-specific surface antigens in T. brucei

The variant specific surface glycoprotein (VSG) genes of T. brucei are expressed in telomeric expression sites. We have determined the structure of the active site in trypanosome variant 221a, which contains VSG gene 221, by analysis of cloned DNA segments that represent 65 kb of the 5'-flanking region of the VSG gene. In nuclear run-on experiments, 57 kb of adjacent sequences are cotranscribed with the VSG gene at approximately similar rates and in the alpha-amanitin-resistant manner characteristic of VSG genes. Besides the VSG mRNA, this expression site yields at least seven stable RNAs, suggesting that it is a multicistronic transcription unit. Our results also show that insertion of a transcriptional terminator is not the general mechanism of switching off expression sites.

Trypanosoma brucei variant-specific glycoprotein gene chromatin is sensitive to single-strand-specific endonuclease digestion

Active variant surface glycoprotein (VSG) gene chromatin is preferentially digested by the restriction enzyme HinfI in nuclei of bloodstream variants of Trypanosoma brucei. HinfI sensitivity of VSG gene chromatin is not observed in nuclei of relapse variants in which the VSG gene has been inactivated in situ. Active VSG gene chromatin is preferentially degraded by the single-strand-specific endonucleases S1 and Bal31. This sensitivity is not the result of pre-existing single-strand breaks or a detectably altered nucleosomal organization. Trypanosome nuclei in which the run-on transcription of VSG genes has been specifically shut down have been used to show that Hinfl and Bal31 sensitivity is not dependent upon continued transcription of the VSG gene. The presence of single-stranded DNA regions within VSG gene chromatin is consistent with a model in which VSG genes are activated by increased torsional stress.

VSG gene 118 is transcribed from a cotransposed pol I-like promoter

To understand the control of differential variant cell surface glycoprotein (VSG) gene expression in T. brucei, we studied VSG gene and expression site transcription regulation. We show that the interchromosomal duplicative transposition of VSG gene 118, on an unusually large transposed segment, results in the transcriptional activation of a cotransposed RNA polymerase I-like (pol I) promoter, from which the VSG gene is transcribed. Transcription of VSG genes by pol I can therefore be regulated by DNA rearrangements that affect positional control of gene expression. A 5' cap is added in trans to the pol I-derived pre-mRNA, by addition of a pol II-derived 35 nucleotide mini-exon. A second gene (ESAG1) is located 25 kb upstream of the VSG 118 gene and is also transcribed. This expression site therefore contains at least two independently regulated genes. We discuss the putative importance of a nucleolar location for VSG gene and expression site transcription regulation.

Frequent independent duplicative transpositions activate a single VSG gene

The expression of several surface antigen genes in Trypanosoma brucei is mediated by the duplicative transposition of a basic-copy variant surface glycoprotein (VSG) gene into an expression site. We determined that the appearance of variant 118, in a parasitemia, resulted from at least four independent duplicative transpositions of the same VSG 118 gene. Variants 117 and 118 both appeared at specific periods but resulted from multiple independent activations. Antigenic variants thus occur in an ordered manner. We show that in the duplicative transpositions of VSG genes, the ends of the transposed segments were homologous between the basic copy and the expression site. Sequences other than the previously reported 70-base-pair (bp) repeats could be involved. In one variant, 118 clone 1, the homology was between a sequence previously transposed into the expression site and a sequence located 6 kilobases upstream of the VSG 118 gene. In variant 118b the homology was presumably in 70-bp repeat arrays, while in a third 118 variant yet another sequence was involved. The possibility that the 70-bp repeats are important in the initial steps of the recombinational events was illustrated by a rearrangement involving a 70-bp repeat array. The data provide strong evidence for the notion that gene conversion mediates the duplicative transposition of VSG genes. We discuss a model that explains how the process of duplicative transposition can occur at random and still produce an ordered appearance of variants.

RNA end-labeling and RNA ligase activities can produce a circular rRNA in whole cell extracts from trypanosomes

We have found two enzymatic activities in whole cell extracts from Trypanosoma brucei; an end-labeling reaction involving a single uridine at the 3' end of ribosomal RNAs (rRNAs) and an RNA ligase activity joining 5' monophosphates to 3' hydroxyl groups. The RNA ligase acts upon one of the small rRNAs (180 nucleotides) from the trypanosome ribosomal repeat unit, forming a circular RNA. The specific circularization of this small rRNA is probably dependent on the secondary structure of the molecule and is not detectable in vivo.

Coincident multiple activations of the same surface antigen gene in Trypanosoma brucei

Trypanosomes with a coat of variant surface glycoprotein (VSG) 118, consistently appear around day 20 when a rabbit is infected with Trypanosoma brucei strain 427. There is a single chromosome-internal gene for VSG 118 and this is activated by duplicative transposition to a telomeric expression site. We show here that the expression-linked extra copy of VSG gene 118 in a day 18 population of a chronic infection is heterogeneous, and we infer that the population is not monoclonal but is the result of multiple independent activations of the 118 gene. We show that the heterogeneity of expression-linked extra copies is also present in other trypanosome populations expressing chromosome-internal VSG genes. We present a model for the timing of VSG gene activation during chronic infection that emphasizes two features: the relative activation and inactivation frequencies of different expression sites, and the degree of homology of the sequences flanking VSG genes with expression sites.

Conserved sequences and transcription of the hsp70 gene family in Trypanosoma brucei

Five Trypanosoma brucei 70-kilodalton heat shock protein-encoding genes (hsp70 genes) were found to be arranged in a tandem array. These hsp70 genes are separated by highly conserved intergenic region sequences of 200 base pairs for one gene and 234 base pairs for the other four genes. This intergenic region sequence is also present in front of the first gene of the tandem array, though at a further distance. All five conserved intergenic regions have sequences that are homologous to the eucaryotic control elements, essential for temperature-induced initiation of transcription by polymerase II. In addition, there is a T-rich region at the 3' end of the hsp70 genes which is homologous to the site of transcription termination of mini-exon genes. Immediately 3' of a putative TATA box, a branch point consensus sequence and six sequences homologous to known trypanosome 3' splice sites were found. It is therefore possible that a PolII promoter is present in the intergenic region sequence. Addition of the 35-nucleotide mini-exon to the hsp70 transcript could thus be mediated by bimolecular splicing. The importance of temperature control for development was illustrated by the response of variant surface glycoprotein-encoding genes to heat shock.

Programmed gene rearrangements altering gene expression

Programmed gene rearrangements are used in nature to to alter gene copy number (gene amplification and deletion), to create diversity by reassorting gene segments (as in the formation of mammalian immunoglobulin genes), or to control the expression of a set of genes that code for the same function (such as surface antigens). Two major mechanisms for expression control are DNA inversion and DNA transposition. In DNA inversion a DNA segment flips around and is rejoined by site-specific recombination, disconnecting or connecting a gene to sequences required for its expression. In DNA transposition a gene moves into an expression site where it displaces its predecessor by gene conversion. Gene rearrangements altering gene expression have mainly been found in some unicellular organisms. They allow a fraction of the organisms to preadapt to sudden changes in environment, that is, to alter properties such as surface antigens in the absence of an inducing stimulus. The antigenic variation that helps the causative agents of African trypanosomiasis, gonorrhea, and relapsing fever to elude host defense is controlled in this way.

Characterization of genes coding for two major metacyclic surface antigens in Trypanosoma brucei

In African trypanosomes, only a very small fraction of the total repertoire of variable antigen types (VATs) is expressed by the metacyclic form. In Trypanosoma brucei stock EATRO 1125, the VATs AnTat 1.30 and 1.45 are reproducibly present in about 15% and 4% of the metacyclic population, respectively. The genes encoding the corresponding antigens or variant surface glycoproteins (VSGs) are in telomeres of large chromosomes, as are some non-metacyclic VSG genes from the same stock. Their activation mechanism has been studied in seven independent clones, 3 of which, referred to as 'first wave' metacyclic VATs (M-VATs), have been cloned from the first wave of parasitemia after cyclic transmission. In all these clones, activation of the antigen gene was linked to the transposition of an expression linked copy (ELC) of the gene to a telomeric expression site. For first wave M-VATs, this site seems variable, although restricted to large chromosomes, and it can be re-used for VSG gene expression in the bloodstream form. In 'late bloodstream' M-VATs, isolated from established chronic infections, the active expression site, at the end of a 200 kb chromosome, is the one preferred for the expression of late antigen types. It can be concluded that no characteristic feature in the genomic location and expression mechanism can distinguish metacyclic antigen genes from those expressed in the bloodstream forms, although the control of their expression must clearly be different.