Maintaining the protective variant surface glycoprotein coat of African trypanosomes

The African trypanosome Trypanosoma brucei has a precarious existence as an extracellular parasite of the mammalian bloodstream, where it is faced with continuous immune attack. Key to survival is a dense VSG (variant surface glycoprotein) coat, which is repeatedly switched during the course of a chronic infection. New data demonstrate a link between VSG synthesis and cell cycle progression, indicating that VSG is monitored during the trypanosome cell cycle.

Structure determination of the human protective protein: twofold averaging reveals the three-dimensional structure of a domain which was entirely absent in the initial model

Mutations in the human 'protective protein' result in the human lysosomal storage disease galactosialidosis. The structure of the human 'protective protein' has been determined using X-ray crystallography to a resolution of 2.2 A. Initial phases were obtained from molecular replacement calculations. A very partial search model comprising 30% of the scattering mass, was constructed from the atomic model of the wheat serine carboxypeptidase. This truncated probe was used to find the position of two monomers in the asymmetric unit. Subsequently, 'bootstrapping' cycles, consisting of twofold averaging and model expansion, retrieved the electron density for residues initially missing. In particular, it proved possible to add a domain (more than 110 residues) to the initial partial search model. In total, 314 residues per asymmetric unit were added to the 588 residues of the initial model. Factors contributing to our success are discussed.

'MAD'ly phasing the extracellular domain of the LDL receptor: a medium-sized protein, large tungsten clusters and multiple non-isomorphous crystals

The crystal structure of the extracellular domain of the LDL receptor (LDL-R) was determined in a MAD experiment using 12-tungstophosphate clusters as anomalous scatterers. While useful for phasing, the tungsten clusters rendered the crystals radiation-sensitive and non-isomorphous and profoundly altered the diffraction data, causing complications. The work is presented as a case study for phasing a medium-sized protein (700 residues) at low resolution (4 A) with multiple non-isomorphous crystals containing 31 W atoms in the asymmetric unit.

LG/LNS domains: multiple functions -- one business end?

The three-dimensional structures of LG/LNS domains from neurexin, the laminin alpha 2 chain and sex hormone-binding globulin reveal a close structural relationship to the carbohydrate-binding pentraxins and other lectins. However, these LG/LNS domains appear to have a preferential ligand-interaction site distinct from the carbohydrate-binding sites found in lectins, and this interaction site accommodates not only sugars but also steroids and proteins. In fact, the LG/LNS domain interaction site has features reminiscent of the antigen-combining sites in immunoglobulins. The LG/LNS domain presents an interesting case in which the fold has remained conserved but the functional sites have evolved; consequently, making predictions of structure-function relationships on the basis of the lectin fold alone is difficult.

The polymorphic telomeres of the African Trypanosome trypanosoma brucei

African trypanosomes have plastic genomes with extensive variability at the chromosome ends. The genes encoding the expressed major surface protein of the infective bloodstream form stages of Trypanosoma brucei and are located at telomeres. These telomeric expression-site transcription units are turning out to be surprisingly polymorphic in structure and sequence.

Genes involved in phenotypic and antigenic variation in African trypanosomes and malaria

Large polymorphic gene families that are involved in clonal phenotypic variation have been identified in both African trypanosomes and malaria parasites. Many of these gene families are necessary for host adaptation, allowing the parasite to infect different species of host or types of host cells. In many cases, switching between these functionally variable proteins also results in antigenic variation.

The structure of the ligand-binding domain of neurexin Ibeta: regulation of LNS domain function by alternative splicing

Neurexins are expressed in hundreds of isoforms on the neuronal cell surface, where they may function as cell recognition molecules. Neurexins contain LNS domains, folding units found in many proteins like the G domain of laminin A, agrin, and slit. The crystal structure of neurexin Ibeta, a single LNS domain, reveals two seven-stranded beta sheets forming a jelly roll fold with unexpected structural similarity to lectins. The LNS domains of neurexin and agrin undergo alternative splicing that modulates their affinity for protein ligands in a neuron-specific manner. These splice sites are localized within loops at one edge of the jelly roll, suggesting a distinct protein interaction surface in LNS domains that is regulated by alternative splicing.

Control of variant surface glycoprotein gene-expression sites in Trypanosoma brucei

Trypanosoma brucei has 20 similar telomeric-expression sites for variant surface glycoprotein genes. Expression sites appear to be controlled at the level of transcription initiation, resulting in only one site being active at any time. Switching between expression sites occurs at a low rate. To analyse the switching mechanism, we used trypanosomes with two expression sites tagged with two different drug-resistance genes and selected these on agarose plates containing both drugs. Double-resistant clones arose at a low frequency of 10(-7) per cell, but these behaved as if they rapidly switched between the two tagged expression sites and lost double resistance in the absence of selection. Using in situ hybridization we found that only 10% of the double-resistant cells had two fluorescent spots corresponding to transcribed expression sites. Our results suggest that: (i) a double expressor is not a stable intermediate in expression site switching; (ii) expression sites are not independently switched on and off; and (iii) expression sites can be in a 'pre-active' silent state from which they can be readily activated.

Mechanisms mediating antigenic variation in Trypanosoma brucei

Antigenic variation in Trypanosoma brucei is a highly sophisticated survival strategy involving switching between the transcription of one of an estimated thousand variant surface glycoprotein (VSG) genes. Switching involves either transcriptional control, resulting in switching between different VSG expression sites; or DNA rearrangement events slotting previously inactive VSG genes into an active VSG expression site. In recent years, considerable progress has been made in techniques allowing us to genetically modify infective bloodstream form trypanosomes. This is allowing us to reengineer VSG expression sites, and look at the effect on the mechanisms subsequently used for antigenic variation. We can now begin a dissection of a highly complicated survival strategy mediated by many different mechanisms operating simultaneously.

Selection for activation of a new variant surface glycoprotein gene expression site in Trypanosoma brucei can result in deletion of the old one

The African trypanosome Trypanosoma brucei expresses the active variant surface glycoprotein (VSG) gene in a telomeric VSG gene expression site. We have generated trypanosomes with a neomycin resistance gene inserted behind an active VSG gene expression site promoter, and a hygromycin resistance gene behind a silent one. By alternating drug selection, we could select for trypanosomes that had switched between the two marked VSG gene expression sites. Surprisingly, trypanosomes that had activated a new VSG gene expression site had often lost the old one. Using polymerase chain reaction (PCR), we screened large numbers of switched trypanosomes and found that sequences lost invariably included the drug marker near the promoter, as well as the telomeric VSG gene many tens of kilobases away. We postulate that stable activation of a new expression site requires silencing of the old one. If silencing does not occur at a sufficient rate by normal switch-off, stable activation of the new site can only occur if the old site is lost in random deletion events. The fact that we pick up these normally infrequent deletions, indicates that inactivation of the old VSG expression site could be rate limiting during switching in our strain of T. brucei.

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

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

Control of VSG gene expression sites in Trypanosoma brucei

Antigenic variation in African trypanosomes continues to be one of the most elaborate and intriguing strategies ever devised by a protozoan parasite to avoid complete destruction by the immune defense of its mammalian host. Here we review some of the recent advances in our understanding of this strategy, concentrating on (unpublished) work from our laboratory.

The atomic model of the human protective protein/cathepsin A suggests a structural basis for galactosialidosis

Human protective protein/cathepsin A (PPCA), a serine carboxypeptidase, forms a multienzyme complex with beta-galactosidase and neuraminidase and is required for the intralysosomal activity and stability of these two glycosidases. Genetic lesions in PPCA lead to a deficiency of beta-galactosidase and neuraminidase that is manifest as the autosomal recessive lysosomal storage disorder galactosialidosis. Eleven amino acid substitutions identified in mutant PPCAs from clinically different galactosialidosis patients have now been modeled in the three-dimensional structure of the wild-type enzyme. Of these substitutions, 9 are located in positions likely to alter drastically the folding and stability of the variant protein. In contrast, the other 2 mutations that are associated with a more moderate clinical outcome and are characterized by residual mature protein appeared to have a milder effect on protein structure. Remarkably, none of the mutations occurred in the active site or at the protein surface, which would have disrupted the catalytic activity or protective function. Instead, analysis of the 11 mutations revealed a substantive correlation between the effect of the amino acid substitution on the integrity of protein structure and the general severity of the clinical phenotype. The high incidence of PPCA folding mutants in galactosialidosis reflects the fact that a single point mutation is unlikely to affect both the beta-galactosidase and the neuraminidase binding sites of PPCA at the same time to produce the double glycosidase deficiency. Mutations in PPCA that result in defective folding, however, disrupt every function of PPCA simultaneously.

Mechanisms of antigenic variation in African trypanosomes

African trypanosomes can escape destruction by the immune system of their mammalian host by antigenic variation of the trypanosome surface coat. This coat is mainly composed of a single protein species, the Variant Surface Glycoprotein or VSG. The genes for VSGs are expressed in a polycistronic telomeric expression site together with at least eight expression site-associated genes (ESAGs). Trypanosomes may switch coat either by replacing the VSG gene in the active expression site by a different one, or by activating another expression site with concomitant silencing of the previously active one. Here we review our present knowledge of antigenic variation in Trypanosome brucei. We focus on four questions: How do trypanosomes switch from one VSG gene expression site to another one? What is the role of the novel base J in silencing expression sites? What is the functional significance of the antigenic variation of the heterodimeric transferrin receptor encoded by two ESAG genes? Why do trypanosomes have multiple expression sites at all?

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.

Telomere exchange can be an important mechanism of variant surface glycoprotein gene switching in Trypanosoma brucei

Trypanosoma brucei undergoes antigenic variation by changing its Variant Surface Glycoprotein (VSG) coat. Although there are up to a thousand VSG genes, only one is transcribed at a time from a telomeric VSG expression site. Switching can involve DNA rearrangements exchanging the active VSG gene, or transcriptional activation of a new expression site and transcriptional silencing of the old one. Determining the mechanism mediating a switch has not always been easy, as the many virtually identical copies of VSG gene expression sites complicate transcriptional analysis. To overcome this problem, we have used bloodstream form T. brucei with a single copy VSG gene in an active expression site marked with a hygromycin resistance gene. We allowed these transformants to undergo switching of the active VSG gene, via three different experimental methods. We were able to select large numbers of switched trypanosomes from a single infected mouse using a new microtitre-dish based procedure developed for this purpose. The drug sensitivity of the switched trypanosomes allowed us to determine the transcriptional state of the marked expression site, and polymerase chain reaction (PCR) amplification was used to determine whether the single copy drug resistance gene and VSG gene present in the marked expression site had been retained. These studies showed that telomere exchange, which has been considered rare, can in some cases be an important mechanism of VSG gene switching. We describe 4 telomere exchange events between the active VSG 221 expression site and 4 different chromosomes.

The viral thymidine kinase gene as a tool for the study of mutagenesis in Trypanosoma brucei

We have tested the use of thymidine kinase as a negative selection system for Trypanosoma brucei. To this end we have targeted a construct containing a Herpes simplex virus thymidine kinase (TK) gene into the ribosomal DNA array of procyclic T. brucei. This resulted in TK activity 30-50-fold above background and in susceptibility to the nucleoside analogues ganciclovir, ethyl-deoxyuridine and 1-[2-deoxy,2-fluoro-8-D-arabinofuranosyl]-5-iodouracil, all of which have no effect on wild-type trypanosomes. TK+ trypanosomes, however, reverted to a ganciclovir resistant phenotype at a rate of 10(-6) per cell-generation. A similar reversion rate was observed using the Varicella-zoster virus TK gene. Loss of TK activity was not due to detectable DNA rearrangements or a decrease in TK mRNA. Sequence analysis of the revertant genes demonstrated, however, the occurrence of point mutations and frameshifts. One revertant line had a mutation in the thymidine binding site leading to the substitution of a conserved arginine by a glycine. Other mutations included single base insertion, single base deletion and the introduction of a premature termination codon by point mutation.

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.

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

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

Three-dimensional structure of the human 'protective protein': structure of the precursor form suggests a complex activation mechanism

BACKGROUND

The human 'protective protein' (HPP) forms a multi-enzyme complex with beta-galactosidase and neuraminidase in the lysosomes, protecting these two glycosidases from degradation. In humans, deficiency of HPP leads to the lysosomal storage disease galactosialidosis. Proteolytic cleavage of the precursor form of HPP involves removal of a 2 kDa excision peptide and results in a carboxypeptidase activity. The physiological relevance of this activity is, as yet, unknown.

RESULTS

The crystal structure of the 108 kDa dimer of the precursor HPP has been elucidated by making extensive use of twofold density averaging. The monomer consists of a 'core' domain and a 'cap' domain. Comparison with the distantly related wheat serine carboxypeptidase dimer shows that the two subunits in the HPP dimer differ by 15 degrees in mutual orientation. Also, the helical subdomain forming part of the cap domains is very different. In addition, the HPP precursor cap domain contains a 'maturation' subdomain of 49 residues which fills the active-site cleft. Merely removing the 'excision' peptide located in the maturation subdomain does not render the catalytic triad solvent accessible.

CONCLUSIONS

The activation mechanism of HPP is unique among proteases with known structure. It differs from the serine proteases in that the active site is performed in the zymogen, but is blocked by a maturation subdomain. In contrast to the zinc metalloproteases and aspartic proteases, the chain segment physically rendering the catalytic triad solvent inaccessible in HPP is not cleaved off to form the active enzyme. The activation must be a multi-step process involving removal of the excision peptide and major conformational changes of the maturation subdomain, whereas the conformation of the enzymatic machinery is probably almost, or completely, unaffected.

Genomic organization of an invariant surface glycoprotein gene family of Trypanosoma brucei

The genomic organization of a gene family for the invariant surface glycoprotein, ISG75 (invariant surface glycoprotein with a molecular mass of 75 kDa), from Trypanosoma brucei is described. In T. brucei strain 427 ISG75 genes are present in tandem arrays at two loci, A and B, containing 5 and 2 copies, respectively. At the 3'-end of locus A, a single gene was identified that encodes a structural isoform of ISG75. This isoform contains a unique amino-terminal domain, whereas the rest of the protein is nearly identical to the polypeptides encoded by the other genes. This isoform is transcribed into a stable mRNA, but the expression of the derived polypeptide was below the detection limit. The ISG75 gene clusters are present on chromosomal bands 9' and 10, supporting the hypothesis of Gottesdiener et al. [25] that these bands contain allelic chromosomes. The total number of ISG75 genes is strain dependent, but at least one copy of the unique isoform is present in every variant tested.

VSG gene expression site control in insect form Trypanosoma brucei

When the African trypanosome Trypanosoma brucei is taken up from mammals by a tse-tse fly, it replaces its variant surface glycoprotein (VSG) coat by a procyclin coat. Transcription of VSG genes stops in the fly, but transcription of sequences derived from the promoter area of the VSG expression site(s) remains high. Whether this is due to continuing high activity of one promoter or to low activity of many promoters was unclear. We have used the small differences between the sequences of different expression sites to show that multiple expression site promoters are active in insect form trypanosomes. This is confirmed by the low expression of single copy marker genes introduced into the transcribed area. However, if the expression site promoter is removed from the genomic location of the expression site and inserted in the non-transcribed spacer of the ribosomal DNA (rDNA), it is derepressed. Derepression of transcription can also be accomplished by replacing the promoter of an expression site by an rDNA promoter. We conclude that the down-regulation of VSG gene expression site promoters in insect form trypanosomes is affected by both the DNA sequence of the promoter and the genomic context in which it resides.

N-glycosylation and deletion mutants of the human MDR1 P-glycoprotein

P-glycoproteins are heavily glycosylated plasma membrane proteins, which confer multidrug resistance by pumping a range of different drugs from the cell. To investigate the significance of the conserved N-glycosylation sites present in the putative first extracellular loop of P-glycoproteins, we mutated one, two, or all three of these sites present in the human MDR1 P-glycoprotein. We also deleted a stretch of 20 amino acids, containing two of the three N-glycosylation sites. The effects of these mutations were studied by transfection into drug-sensitive cells. In vincristine-resistant transfected clones selected for similar steady state levels of membrane-bound P-glycoprotein, the absence of N-glycosylation did not alter the level or pattern of (cross-)resistance. However, the absence of N-glycosylation sites drastically reduced the efficiency with which drug-resistant clones could be generated. These findings suggest that N-glycosylation contributes to proper routing or stability of P-glycoprotein but not to drug transport per se. The deletion mutants demonstrated a clearly decreased and altered drug resistance pattern, even with a high level of P-glycoprotein in the plasma membrane. This, and possibly the observed lack of glycosylation of the remaining intact glycosylation sequence, suggests a constrained P-glycoprotein structure. Our findings support the current model for P-glycoprotein structure.

In search of new lead compounds for trypanosomiasis drug design: a protein structure-based linked-fragment approach

A modular method for pursuing structure-based inhibitor design in the framework of a design cycle is presented. The approach entails four stages: (1) a design pathway is defined in the three-dimensional structure of a target protein; (2) this pathway is divided into subregions; (3) complementary building blocks, also called fragments, are designed in each subregion; complementarity is defined in terms of shape, hydrophobicity, hydrogen bond properties and electrostatics; and (4) fragments from different subregions are linked into potential lead compounds. Stages (3) and (4) are qualitatively guided by force-field calculations. In addition, the designed fragments serve as entries for retrieving existing compounds from chemical databases. This linked-fragment approach has been applied in the design of potentially selective inhibitors of triosephosphate isomerase from Trypanosoma brucei, the causative agent of sleeping sickness.

The PARP and VSG genes of Trypanosoma brucei do not resemble RNA polymerase II transcription units in sensitivity to Sarkosyl in nuclear run-on assays

Addition of the ionic detergent N-lauroylsarcosine (Sarkosyl) affects the efficiency of transcription of genes of the protozoan Trypanosoma brucei in nuclear run-on assays. Transcription of the PARP (procyclin or procyclic acidic repetitive protein), variant cell surface glycoprotein (VSG) and ribosomal RNA (rRNA) genes was resistant or increased after addition of Sarkosyl. In contrast, the transcription of seven protein coding house keeping genes and the mini-exon donor RNA (medRNA) genes was completely abolished by the addition of Sarkosyl, while the transcription of the 5S rRNA genes showed an intermediate sensitivity. We conclude that Sarkosyl can be used to discriminate between the different types of trypanosome transcription units. The PARP and VSG protein coding genes had previously been postulated to be transcribed by an RNA polymerase I-like enzyme on the basis of their resistance to the RNA polymerase II inhibitor alpha-amanitin. This model is now supported by their resistance to the addition of Sarkosyl.

RNA polymerase I can mediate expression of CAT and neo protein-coding genes in Trypanosoma brucei

We show that the ribosomal RNA (rRNA) promoter can efficiently direct expression of protein-coding genes in the parasitic protozoan Trypanosoma brucei. The rRNA promoter was characterized by: (i) point mutations at the rRNA transcription initiation site which completely abolished its promoter function in transient CAT transformation assays; (ii) the alpha-amanitin resistance of transcription of rRNA promoter-neomycin phosphotransferase (neo) genes in stably transformed trypanosomes; and (iii) the nucleolar location of neo RNA, synthesized under the control of the rRNA promoter. The rRNA promoter-derived CAT mRNA required a 3' splice acceptor site and the neo mRNA was trans-spliced and polyadenylated. In situ hybridization revealed neo RNA at the nucleolus in stably transformed trypanosomes in which rRNA promoter-neo constructs were integrated either at a rRNA locus or at a locus for the procyclic acidic repetitive protein (PARP) coding genes. We postulate that trans-splicing, by uncoupling the requirement for transcription of protein-coding genes by RNA polymerase II, allows RNA polymerase I mediated protein-coding gene transcription, presumably because a 5' cap can be transferred to the pre-mRNA by trans-splicing.

Procyclic acidic repetitive protein (PARP) genes located in an unusually small alpha-amanitin-resistant transcription unit: PARP promoter activity assayed by transient DNA transfection of Trypanosoma brucei

At least one of the procyclic acidic repetitive protein (PARP or procyclin) loci of Trypanosoma brucei is a small (5- to 6-kilobase) polycistronic transcription unit which is transcribed in an alpha-amanitin-resistant manner. Its single promoter, as mapped by run-on transcription analysis and UV inactivation of transcription, is located immediately upstream of the first alpha-PARP gene. Transcription termination occurs in a region approximately 3 kilobases downstream of the beta-PARP gene. The location of the promoter was confirmed by its ability to direct transcription of the bacterial chloramphenicol acetyltransferase gene in insect-form (procyclic) T. brucei. The putative PARP promoter is located in the region between the 3' splice acceptor site (nucleotide position 0) and nucleotide position -196 upstream of the alpha-PARP genes. Regulatory regions influencing the levels of PARP expression may be located further upstream. We conclude that a single promoter, which is located very close to the 3' splice acceptor site of the alpha-PARP genes, directs the transcription of a small, polycistronic, and alpha-amanitin-resistant transcription unit.

Alpha-amanitin resistant transcription of protein coding genes in insect and bloodstream form Trypanosoma brucei

The variant cell surface glycoprotein (VSG) gene expression sites of the protozoan Trypanosoma brucei are transcribed by an unusual alpha-amanitin resistant RNA polymerase. All other protein coding genes of T.brucei examined to date are transcribed by an alpha-amanitin sensitive RNA polymerase, presumably RNA polymerase II. We now show that transcription of protein coding genes by alpha-amanitin resistant RNA polymerases is not unique to the bloodstream form expressed VSG gene expression sites, but also occurs in insect form trypanosomes, which do not express VSG genes. In insect form trypanosomes transcription of the procyclin or PARP genes is resistant to alpha-amanitin to a degree comparable with that of VSG and ribosomal RNA (rRNA) genes. Comparison of the alpha-amanitin resistantly transcribed PARP and VSG gene families shows that they both produce one of the most abundant mRNAs [1-3% of poly(A)+] and they both encode the major cell surface proteins of their respective life cycle stages. Transcription of a subset of functionally comparable protein coding genes is thus mediated by an RNA polymerase which differs from the regular RNA polymerase II.

Transcription of telomere repeats in protozoa

The telomerically located variant cell surface glycoprotein (VSG) gene expression sites of the protozoan parasite Trypanosoma brucei are transcribed by an unusual alpha-amanitin resistant RNA polymerase. We show that the telomere GGGTTA repeats located at the chromosome ends of T. brucei and the related protozoan T. equiperdum are also transcribed by alpha-amanitin resistant RNA polymerases. This transcription predominantly proceeds unidirectionally towards the end of the chromosome, in both bloodstream and insect form trypanosomes and results in the generation of heterogeneously sized steady state RNA. We postulate that telomere repeat transcription results from readthrough downstream of telomeric genes. Telomere repeat transcription was found in all seven protozoan species tested, but was alpha-amanitin resistant only in trypanosome species which exhibited antigenic variation. The data indicate that in some trypanosome species a subset of telomeres is transcribed by a different type of RNA polymerase.

Crosses of two independently derived transgenic mice demonstrate functional complementation of the genes encoding heavy (HLA-B27) and light (beta 2-microglobulin) chains of HLA class I antigens

In man a number of diseases are associated with certain alleles of MHC antigens. The most pronounced example is ankylosing spondylitis, which is strongly associated with HLA-B27. As a first step towards a model system to study the basis of this association, transgenic mice were generated that showed cell surface expression of the HLA-B27 antigen biochemically indistinguishable from HLA-B27 antigen expressed on human cells. This result was obtained by crossing two independently derived strains of mice, one of which is transgenic for the HLA-B27 heavy chain gene, and the other carrying and expressing the human beta 2m gene. Examination of HLA-B27 and human beta 2m mRNA in various tissues shows the two genes to be expressed in a coordinate fashion. The mRNA levels follow those of endogenous H-2 Class I genes.

Three small RNAs within the 10 kb trypanosome rRNA transcription unit are analogous to domain VII of other eukaryotic 28S rRNAs

We have localized the six ribosomal RNAs (rRNAs) which encode the 28S rRNA region of Trypanosoma brucei. These six rRNAs include two large rRNAs, 28S alpha (approx. 1840 nt) and 28S beta (approx. 1570 nt), and four small rRNAs of approximate sizes 220, 180, 140 and 70 nt. Three of these four small rRNAs (180, 70 and 140) are found at the 3' end of the 28S rRNAs region. Sequence analysis of this area shows that these three small rRNAs encode Domain VII, the last domain of secondary structure in the 28S rRNAs of eukaryotes. Hybridization of labeled nascent RNA to the cloned repeat unit and S1 nuclease protection analysis of putative precursors show that transcription initiates approximately 1.2 kb upstream of the 18S rRNA and terminates after the last small rRNA (140) at the 3' end of the 28S rRNA region. Analysis of three putative rRNA precursors suggests that the small rRNAs are not processed from the primary transcript until after the usual processing of the 5.8S rRNA region.