Transcription initiation is controlled by three core promoter elements in the hgl5 gene of the protozoan parasite Entamoeba histolytica

Entamoeba histolytica is a single cell eukaryote that is the etiologic agent of amoebic colitis. Core promoter elements of E. histolytica protein encoding genes include a TATA-like sequence (GTATTTAAAG/C) at −30, a novel element designated GAAC (GAACT) that has a variable location between TATA and the site of transcription initiation, and a putative initiator (Inr) element (AAAAATTCA) overlying the site of transcription initiation. The presence of three separate conserved sequences in a eukaryotic core promoter is unprecedented and prompted examination of their roles in regulating transcription initiation. Alterations of all three regions in the hgl5 gene decreased reporter gene activity with the greatest effect seen by mutation of the GAAC element. Positional analysis of the TATA box demonstrated that transcription initiated consistently 30–31 bases downstream of the TATA region. Mutation of either the TATA or GAAC elements resulted in the appearance of new transcription start sites upstream of +1 in the promoter of the hgl5 gene. Mutation of the Inr element resulted in no change in the site of transcription initiation; however, in the presence of a mutated TATA and GAAC regions, the Inr element controlled the site of transcription initiation. We conclude that all three elements play a role in determining the site of transcription initiation. The variable position of the GAAC element relative to the site of transcription initiation, and the multiple transcription initiations that resulted from its mutation, indicate that the GAAC element has an important and apparently novel role in transcriptional control in E. histolytica.

Telomeres of polytene chromosomes in a ciliated protozoan terminate in duplex DNA loops

The end of a telomeric DNA sequence isolated from a polytene chromosome of a hypotrichous ciliate folds back and hybridizes with downstream telomeric sequence to form a t loop that is stable in the absence of protein and DNA cross-linking. The single-stranded, telomeric DNA sequence at the end of a macronuclear molecule does not form a t loop but, instead, is complexed with a heterodimeric, telomere-binding protein. Thus, two mechanisms for capping the ends of DNA molecules are used in the same cell.

Surface antigen cross-linking triggers forced exit of a protozoan parasite from its host.

We used the common fish pathogen Ichthyophthirius multifiliis as a model for studying interactions between parasitic ciliates and their vertebrate hosts. Although highly pathogenic, Ichthyophthirius can elicit a strong protective immune response in fish after exposure to controlled infections. To investigate the mechanisms underlying host resistance, a series of passive immunization experiments were carried out using mouse monoclonal antibodies against a class of surface membrane proteins, known as immobilization antigens (or i-antigens), thought to play a role in the protective response. Such antibodies bind to cilia and immobilize I. multifiliis in vitro. Surprisingly, we found that passive antibody transfer in vivo caused rapid exit of parasites from the host. The effect was highly specific for a given I. multifiliis serotype. F(ab)2 subfragments had the same effect as intact antibody, whereas monovalent Fab fragments failed to protect. The activity of Fab could, nevertheless, be restored after subsequent i.p. injection of bivalent goat anti-mouse IgG. Parasites that exit the host had detectable antibody on their surface and appeared viable in all respects. These findings represent a novel instance among protists in which protective immunity (and evasion of the host response) result from an effect of antibody on parasite behavior.

A cooperative model for receptor recognition and cell adhesion: evidence from the molecular packing in the 1.6-A crystal structure of the pheromone Er-1 from the ciliated protozoan Euplotes raikovi.

The crystal structure of the pheromone Er-1 from the unicellular eukaryotic organism Euplotes raikovi was determined at 1.6 A resolution and refined to a crystallographic R factor of 19.9%. In the tightly packed crystal, two extensive intermolecular helix-helix interactions arrange the Er-1 molecules into layers. Since the putative receptor of the pheromone is a membrane-bound protein, whose extracellular C-terminal domain is identical in amino acid sequence to the soluble pheromone, the interactions found in the crystal may mimic the pheromone-receptor interactions as they occur on a cell surface. Based on this, we propose a model for the interaction between soluble pheromone molecules and their receptors. In this model, strong pheromone-receptor binding emerges as a consequence of the cooperative utilization of several weak interactions. The model offers an explanation for the results of binding studies and may also explain the adhesion between cells that occurs during mating.

Transfection and continuous expression of heterologous genes in the protozoan parasite Entamoeba histolytica.

To provide tools for functional molecular genetics of the protozoan parasite Entamoeba histolytica, we investigated the use of the prokaryotic neomycin phosphotransferase (NEO) gene as a selectable marker for the transfection of the parasite. An Escherichia coli-derived plasmid vector was constructed (pA5'A3'NEO) containing the NEO coding region flanked by untranslated 5' and 3' sequences of an Ent. histolytica actin gene. Preceding experiments had revealed that amoebae are highly sensitive to the neomycin analogue G418 and do not survive in the presence of as little as 2 micrograms/ml. Transfection of circular pA5'A3'NEO via electroporation resulted in Ent. histolytica trophozoites resistant to G418 up to 100 micrograms/ml. DNA and RNA analyses of resistant cells indicated that (i) the transfected DNA was not integrated into the amoeba genome but was segregated episomally, (ii) in the amoebae, the plasmid replicated autonomously, (iii) the copy number of the plasmid and the expression of NEO-specific RNA were proportional to the amount of G418 used for selection, and (iv) under continuous selection, the plasmid was propagated over an observation period of 6 months. Moreover, the plasmid could be recloned into E. coli and was found to be unrearranged. To investigate the use of pA5'A3'NEO to coexpress other genes in Ent. histolytica, a second marker, the prokaryotic chloramphenicol acetyltransferase (CAT) gene under control of an Ent. histolytica lectin gene promoter was introduced into the plasmid. Transfection of the amoebae with this construct also conferred G418 resistance and, in addition, allowed continuous expression of CAT activity in quantities corresponding to the amount of G418 used for selection. When selection was discontinued, transfected plasmids were lost as indicated by an exponential decline of CAT activity in trophozoite extracts.

Insertional mutagenesis and marker rescue in a protozoan parasite: cloning of the uracil phosphoribosyltransferase locus from Toxoplasma gondii.

Nonhomologous integration vectors have been used to demonstrate the feasibility of insertional mutagenesis in haploid tachyzoites of the protozoan parasite Toxoplasma gondii. Mutant clones resistant to 5-fluorouracil were identified at a frequency of approximately 10(-6) (approximately 2 x 10(-5) of the stable transformants). Four independent mutants were isolated, all of which were shown to lack uracil phosphoribosyl-transferase (UPRT) activity and harbor transgenes integrated at closely linked loci, suggesting inactivation of the UPRT-encoding gene. Genomic DNA flanking the insertion point (along with the integrated vector) was readily recovered by bacterial transformation with restriction-digested, self-ligated total genomic DNA. Screening of genomic libraries with the recovered fragment identified sequences exhibiting high homology to known UPRT-encoding genes from other species, and cDNA clones were isolated that contain a single open reading frame predicted to encode the 244-amino acid enzyme. Homologous recombination vectors were exploited to create genetic knock-outs at the UPRT locus, which are deficient in enzyme activity but can be complemented by transient transformation with wild-type sequences--formally confirming identification of the functional UPRT gene. Mapping of transgene insertion points indicates that multiple independent mutants arose from integration at distinct sites within the UPRT gene, suggesting that nonhomologous integration is sufficiently random to permit tagging of the entire parasite genome in a single transformation.

Nuclear-encoded proteins target to the plastid in Toxoplasma gondii and Plasmodium falciparum

A vestigial, nonphotosynthetic plastid has been identified recently in protozoan parasites of the phylum Apicomplexa. The apicomplexan plastid, or “apicoplast,” is indispensable, but the complete sequence of both the Plasmodium falciparum and Toxoplasma gondii apicoplast genomes has offered no clue as to what essential metabolic function(s) this organelle might perform in parasites. To investigate possible functions of the apicoplast, we sought to identify nuclear-encoded genes whose products are targeted to the apicoplast in Plasmodium and Toxoplasma. We describe here nuclear genes encoding ribosomal proteins S9 and L28 and the fatty acid biosynthetic enzymes acyl carrier protein (ACP), β-ketoacyl-ACP synthase III (FabH), and β-hydroxyacyl-ACP dehydratase (FabZ). These genes show high similarity to plastid homologues, and immunolocalization of S9 and ACP verifies that the proteins accumulate in the plastid. All the putatively apicoplast-targeted proteins bear N-terminal presequences consistent with plastid targeting, and the ACP presequence is shown to be sufficient to target a recombinant green fluorescent protein reporter to the apicoplast in transgenic T. gondii. Localization of ACP, and very probably FabH and FabZ, in the apicoplast implicates fatty acid biosynthesis as a likely function of the apicoplast. Moreover, inhibition of P. falciparum growth by thiolactomycin, an inhibitor of FabH, indicates a vital role for apicoplast fatty acid biosynthesis. Because the fatty acid biosynthesis genes identified here are of a plastid/bacterial type, and distinct from those of the equivalent pathway in animals, fatty acid biosynthesis is potentially an excellent target for therapeutics directed against malaria, toxoplasmosis, and other apicomplexan-mediated diseases.

Double-stranded RNA induces mRNA degradation in Trypanosoma brucei

Double-stranded RNA (dsRNA) recently has been shown to give rise to genetic interference in Caenorhabditis elegans and also is likely to be the basis for phenotypic cosuppression in plants in certain instances. While constructing a plasmid vector for transfection of trypanosome cells, we serendipitously discovered that in vivo expression of dsRNA of the α-tubulin mRNA 5′ untranslated region (5′ UTR) led to multinucleated cells with striking morphological alterations and a specific block of cytokinesis. Transfection of synthetic α-tubulin 5′ UTR dsRNA, but not of either strand individually, caused the same phenotype. On dsRNA transfection, tubulin mRNA, but not the corresponding pre-mRNA, was rapidly and specifically degraded, leading to a deficit of α-tubulin synthesis. The transfected cells were no longer capable of carrying out cytokinesis and eventually died. Analysis of cytoskeletal structures from these trypanosomes revealed defects in the microtubules of the flagellar axoneme and of the flagellar attachment zone, a complex cortical structure that we propose is essential for establishing the path of the cleavage furrow at cytokinesis. Last, dsRNA-mediated mRNA degradation is not restricted to α-tubulin mRNA but can be applied to other cellular mRNAs, thus establishing a powerful tool to genetically manipulate these important protozoan parasites.

Parasite-mediated nuclear factor κB regulation in lymphoproliferation caused by Theileria parva infection

Infection of cattle with the protozoan Theileria parva results in uncontrolled T lymphocyte proliferation resulting in lesions resembling multicentric lymphoma. Parasitized cells exhibit autocrine growth characterized by persistent translocation of the transcriptional regulatory factor nuclear factor κB (NFκB) to the nucleus and consequent enhanced expression of interleukin 2 and the interleukin 2 receptor. How T. parva induces persistent NFκB activation, required for T cell activation and proliferation, is unknown. We hypothesized that the parasite induces degradation of the IκB molecules which normally sequester NFκB in the cytoplasm and that continuous degradation requires viable parasites. Using T. parva-infected T cells, we showed that the parasite mediates continuous phosphorylation and proteolysis of IκBα. However, IκBα reaccumulated to high levels in parasitized cells, which indicated that T. parva did not alter the normal NFκB-mediated positive feedback loop which restores cytoplasmic IκBα. In contrast, T. parva mediated continuous degradation of IκBβ resulting in persistently low cytoplasmic IκBβ levels. Normal IκBβ levels were only restored following T. parva killing, indicating that viable parasites are required for IκBβ degradation. Treatment of T. parva-infected cells with pyrrolidine dithiocarbamate, a metal chelator, blocked both IκB degradation and consequent enhanced expression of NFκB dependent genes. However treatment using the antioxidant N-acetylcysteine had no effect on either IκB levels or NFκB activation, indicating that the parasite subverts the normal IκB regulatory pathway downstream of the requirement for reactive oxygen intermediates. Identification of the critical points regulated by T. parva may provide new approaches for disease control as well as increase our understanding of normal T cell function.

Trypanosoma cruzi Calreticulin Is a Lectin That Binds Monoglucosylated Oligosaccharides but Not Protein Moieties of Glycoproteins

Trypanosoma cruzi is a protozoan parasite that belongs to an early branch in evolution. Although it lacks several features of the pathway of protein N-glycosylation and oligosaccharide processing present in the endoplasmic reticulum of higher eukaryotes, it displays UDP-Glc:glycoprotein glucosyltransferase and glucosidase II activities. It is herewith reported that this protozoan also expresses a calreticulin-like molecule, the third component of the quality control of glycoprotein folding. No calnexin-encoding gene was detected. Recombinant T. cruzi calreticulin specifically recognized free monoglucosylated high-mannose-type oligosaccharides. Addition of anti-calreticulin serum to extracts obtained from cells pulse–chased with [35S]Met plus [35S]Cys immunoprecipitated two proteins that were identified as calreticulin and the lysosomal proteinase cruzipain (a major soluble glycoprotein). The latter but not the former protein disappeared from immunoprecipitates upon chasing cells. Contrary to what happens in mammalian cells, addition of the glucosidase II inhibitor 1-deoxynojirimycin promoted calreticulin–cruzipain interaction. This result is consistent with the known pathway of protein N-glycosylation and oligosaccharide processing occurring in T. cruzi. A treatment of the calreticulin-cruzipain complexes with endo-β-N-acetylglucosaminidase H either before or after addition of anti-calreticulin serum completely disrupted calreticulin–cruzipain interaction. In addition, mature monoglucosylated but not unglucosylated cruzipain isolated from lysosomes was found to interact with recombinant calreticulin. It was concluded that the quality control of glycoprotein folding appeared early in evolution, and that T. cruzi calreticulin binds monoglucosylated oligosaccharides but not the protein moiety of cruzipain. Furthermore, evidence is presented indicating that glucosyltransferase glucosylated cruzipain at its last folding stages.

Role of cysteines in Plasmodium falciparum circumsporozoite protein: Interactions with heparin can rejuvenate inactive protein mutants

Various pathogenic bacteria, viruses, and protozoan bind to glycosaminoglycan-based receptors on host cells and initiate an infection. Sporozoites of Plasmodium predominantly express circumsporozoite (CS) protein on their surface, which binds to heparan sulfate proteoglycans on liver cell surface that subsequently leads to malaria. Here we show that the interaction of free heparin with this parasite ligand has the potential to be a critical component of invasion. CS protein of P. falciparum contains four cysteines at positions 361, 365, 396, and 401. In this study, all four cysteine residues were mutagenized to alanine both individually and in different combinations. Conversion of cysteine 396 to alanine (protein CS3) led to a 10-fold increase in the binding activity of the protein to HepG2 cells. Replacement of cysteines at positions 361, 365, and 401 either alone or in different combinations led to a near total loss of binding. Surprisingly, activity in these inactive mutants could be effectively restored in the presence of submolar concentrations of heparin. Heparin also up-regulated binding of CS3 at submolar concentrations with respect to the protein but down-regulated binding when present in excess. Given the significantly different concentrations of heparin in different organs of the host and the in vitro results described here one can consider in vivo ramifications of this phenomenon for pathogen targeting of specific organs and for the functional effects of antigenic variation on receptor ligand interaction.

Critical roles of glycosylphosphatidylinositol for Trypanosoma brucei

Trypanosoma brucei, the protozoan parasite responsible for sleeping sickness, evades the immune response of mammalian hosts and digestion in the gut of the insect vector by means of its coat proteins tethered to the cell surface via glycosylphosphatidylinositol (GPI) anchors. To evaluate the importance of GPI for parasite survival, we cloned and disrupted a trypanosomal gene, TbGPI10, involved in biosynthesis of GPI. TbGPI10 encodes a protein of 558 amino acids having 25% and 23% sequence identity to human PIG-B and Saccharomyces cerevisiae Gpi10p, respectively. TbGPI10 restored biosynthesis of GPI in a mouse mutant cell line defective in mouse Pig-b gene. TbGPI10 also rescued the inviability of GPI10-disrupted S. cerevisiae, indicating that TbGPI10 is the orthologue of PIG-B/GPI10 that is involved in the transfer of the third mannose to GPI. The bloodstream form of T. brucei could not lose TbGPI10; therefore, GPI synthesis is essential for growth of mammalian stage parasites. Procyclic form cells (insect stage parasites) lacking the surface coat proteins because of disruption of TbGPI10 are viable and grow slower than normal, provided that they are cultured in nonadherent flasks. In regular flasks, they adhered to the plastic surface and died. Infectivity to tsetse flies is partially impaired, particularly in the early stage. Therefore, parasitespecific inhibition of GPI biosynthesis should be an effective chemotherapy target against African trypanosomiasis.

The surface coat of procyclic Trypanosoma brucei: Programmed expression and proteolytic cleavage of procyclin in the tsetse fly

Trypanosoma brucei, the protozoan parasite causing sleeping sickness, is transmitted by a tsetse fly vector. When the tsetse takes a blood meal from an infected human, it ingests bloodstream form trypanosomes that quickly differentiate into procyclic forms within the fly's midgut. During this process, the parasite loses the 107 molecules of variant surface glycoprotein that formed its surface coat, and it develops a new coat composed of several million procyclin molecules. Procyclins, the products of a small multigene family, are glycosyl phosphatidylinositol-anchored proteins containing characteristic amino acid repeats at the C terminus [either EP (EP procyclin, a form of procyclin rich in Glu-Pro repeats) or GPEET (GPEET procyclin, a form of procyclin rich in Glu-Pro-Glu-Glu-Thr repeats)]. We have used a sensitive and accurate mass spectrometry method to analyze the appearance of different procyclins during the establishment of midgut infections in tsetse flies. We found that different procyclin gene products are expressed in an orderly manner. Early in the infection (day 3), GPEET2 is the only procyclin detected. By day 7, however, GPEET2 disappears and is replaced by several isoforms of glycosylated EP, but not the unglycosylated isoform EP2. Unexpectedly, we discovered that the N-terminal domains of all procyclins are quantitatively removed by proteolysis in the fly, but not in culture. These findings suggest that one function of the protease-resistant C-terminal domain, containing the amino acid repeats, is to protect the parasite surface from digestive enzymes in the tsetse fly gut.

Cis-acting requirements in flanking DNA for the programmed elimination of mse2.9: a common mechanism for deletion of internal eliminated sequences from the developing macronucleus of Tetrahymena thermophila

During macronuclear development in the ciliated protozoan Tetrahymena thermophila, extensive DNA deletions occur, eliminating thousands of internal eliminated sequences (IESs). Using an rDNA-based transformation assay we have analyzed the role during DNA deletion of DNA flanking mse2.9, an IES within the second intron of a gene encoding an as yet incompletely characterized protein. We establish that a cis-acting sequence for mse2.9 deletion acts at a distance to specify deletion boundaries. A complex sequence element necessary for efficient and accurate mse2.9 deletion is located in the region 47–81 bp from the right side of mse2.9. The ability of a variety of IES flanking sequences to rescue a processing deficient mse2.9 construct indicates that some cis-acting signal is shared among different IESs. In addition, the short intronic sequence that flanks mse2.9 is able to direct efficient and accurate processing. Despite no obvious sequence similarity between mse2.9 and other IESs, we suggest that a common mechanism is used to delete different families of IESs in Tetrahymena.

Characterisation of the gene encoding type II DNA topoisomerase from Leishmania donovani: a key molecular target in antileishmanial therapy

The gene encoding type II DNA topoisomerase from the kinetoplastid hemoflagellated protozoan parasite Leishmania donovani (LdTOP2) was isolated from a genomic DNA library of this parasite. DNA sequence analysis revealed an ORF of 3711 bp encoding a putative protein of 1236 amino acids with no introns. The deduced amino acid sequence of LdTOP2 showed strong homologies to TOP2 sequences from other kinetoplastids, namely Crithidia and Trypanosoma spp. with estimated identities of 86 and 68%, respectively. LdTOP2 shares a much lower identity of 32% with its human homologue. LdTOP2 is located as a single copy on a chromosome in the 0.7 Mb region in the L.donovani genome and is expressed as a 5 kb transcript. 5′-Mapping studies indicate that the LdTOP2 gene transcript is matured post-transcriptionally with the trans-splicing of the mini-exon occurring at –639 from the predicted initiation site. Antiserum raised in rabbit against glutathione S-transferase fusion protein containing the major catalytic portion of the recombinant L.donovani topoisomerase II protein could detect a band on western blots at ∼132 kDa, the expected size of the entire protein. Use of the same antiserum for immunolocalisation analysis led to the identification of nuclear, as well as kinetoplast, antigens for L.donovani topoisomerase II. The in vitro biochemical properties of the full-length recombinant LdTOP2 when overexpressed in E.coli were similar to the Mg(II) and ATP-dependent activity found in cell extracts of L.donovani.