Plasmodium telomeres and telomerase: the usual actors in an unusual scenario

PfGBP2 is a novel G-quadruplex binding protein in Plasmodium falciparum

Guanine-quadruplexes (G4s) are non-canonical DNA structures that can regulate key biological processes such as transcription, replication and telomere maintenance in several organisms including eukaryotes, prokaryotes and viruses. Recent reports have identified the presence of G4s within the AT-rich genome of Plasmodium falciparum, the protozoan parasite causing malaria. In Plasmodium, potential G4-forming sequences (G4FS) are enriched in the telomeric and sub-telomeric regions of the genome where they are associated with telomere maintenance and recombination events within virulence genes. However, there is a little understanding about the biological role of G4s and G4-binding proteins. Here, we provide the first snapshot of G4-interactome in P. falciparum using DNA pull-down assay followed by LC-MS/MS. Interestingly, we identified ~24 potential G4-binding proteins (G4-BP) that bind to a stable G4FS (AP2_G4). Furthermore, we characterised the role of G-strand binding protein 2 (PfGBP2), a putative telomere-binding protein in P. falciparum. We validated the interaction of PfGBP2 with G4 in vitro as well as in vivo. PfGBP2 is expressed throughout the intra-erythrocytic developmental cycle and is essential for the parasites in the presence of G4-stabilising ligand, pyridostatin. Gene knockout studies showed the role of PfGBP2 in the expression of var genes. Taken together, this study suggests that PfGBP2 is a bona fide G4-binding protein, which is likely to be involved in the regulation of G4-related functions in these malarial parasites. In addition, this study sheds light on this understudied G4 biology in P. falciparum.

Identification of GdRFC1 as a novel regulator of telomerase in Giardia duodenalis

Telomerase plays a crucial role in ageing and tumourigenesis. However, the regulatory network of its activity is complicated and not fully understood. In the present study, a yeast two-hybrid screen identified a homologue of human replication factor C subunit 1 (RFC1) as a novel interacting protein of Giardia duodenalis GdTRBD (Giardia duodenalis telomerase ribonucleoprotein complex RNA binding domain GdTRBD). This interaction was further verified via GST pull-down in vitro and co-immunoprecipitation (Co-IP) and bimolecular fluorescence complementation (BiFC) in vivo. We also found that GdRFC1 (Giardia duodenalis replication factor C subunit 1) only interacted with GdTRBD in one nucleus in Giardia duodenalis via a proximity ligation assay (PLA). We reasoned that the two nuclei might have significant heterogeneity in their functional activities during the trophozoite stage and that the two molecules might be involved in other unidentified functions in addition to telomerase activity. In addition, knockdown of GdRFC1 decreased telomerase activity. Collectively, our results indicate that GdRFC1 is a novel binding partner and positive regulator of telomerase in Giardia duodenalis.

Genetic polymorphism of histidine rich protein 2 in Plasmodium falciparum isolates from different infection sources in Yunnan Province, China

Background

Failed diagnoses of some falciparum malaria cases by RDTs are constantly reported in recent years. Plasmodium falciparum histidine-rich protein 2 (pfhpr2) gene deficiency has been found to be the major reason of RDTs failure in many countries. This article analysed the deletion of pfhpr2 gene of falciparum malaria cases isolated in Yunnan Province, China.

Methods

Blood samples from falciparum malaria cases diagnosed in Yunnan Province were collected. Plasmodium genomic DNA was extracted and the pfhrp2 gene exon2 region was amplified via nested PCR. The haplotype of the DNA sequence, the nucleic acid diversity index (PI) and expected heterozygosity (He) were analyzed. Count PfHRP2 amino acid peptide sequence repeat and its times, and predict the properties of PfHRP2 peptide chain reaction to RDTs testing.

Results

A total of 306 blood samples were collected, 84.9% (259/306) from which pfhrp2 PCR amplification products (gene exon2) were obtained, while the remaining 47 samples were false amplification. The length of the 250 DNA sequences ranged from 345 - 927 bp, with 151 haplotypes, with PI and He values of 0.169 and 0.983, respectively. The length of the PfHRP2 peptide chain translated from 250 DNA sequences ranged from 115 to 309 aa. All peptide chains had more than an amino acid codon deletion. All 250 PfHRP2 strands ended with a type 12 amino acid repeat, 98.0% (245/250) started with a type 1 repetition and 2.0% (5/250) with a type 2 repetition. The detection rate for type 2 duplicates was 100% (250/250). Prediction of RDT sensitivity of PfHRP2 peptide chains based on type 2 and type 7 repeats showed that 9.60% (24/250), 50.0% (125/250), 13.20% (33/250) and 27.20.5% (68/250) of the 250 peptide chains were very sensitive, sensitive, borderline and non-sensitive, respectively.

Conclusion

The diversified polymorphism of the pfhrp2 gene deletion from different infection sources in the Yunnan province are extremely complex. The cause of the failure of pfhrp2 exon2 amplification is still to be investigated. The results of this study appeal to Yunnan Province for a timely evaluation of the effectiveness and applicability of RDTs in the diagnosis of malaria.

PhyloChromoMap, a Tool for Mapping Phylogenomic History along Chromosomes, Reveals the Dynamic Nature of Karyotype Evolution in Plasmodium falciparum

The genome of Plasmodium falciparum, the causative agent of malaria in Africa, has been extensively studied since it was first fully sequenced in 2002. However, many open questions remain, including understanding the chromosomal context of molecular evolutionary changes (e.g., relationship between chromosome map and phylogenetic conservation, patterns of gene duplication, and patterns of selection). Here, we present PhyloChromoMap, a method that generates a phylogenomic map of chromosomes from a custom-built bioinformatics pipeline. Using P. falciparum 3D7 as a model, we analyze 2,116 genes with homologs in up to 941 diverse eukaryotic, bacterial and archaeal lineages. We estimate the level of conservation along chromosomes based on conservation across clades, and identify “young” regions (i.e., those with recent or fast evolving genes) that are enriched in subtelomeric regions as compared with internal regions. We also demonstrate that patterns of molecular evolution for paralogous genes differ significantly depending on their location as younger paralogs tend to be found in subtelomeric regions whereas older paralogs are enriched in internal regions. Combining these observations with analyses of synteny, we demonstrate that subtelomeric regions are actively shuffled among chromosome ends, which is consistent with the hypothesis that these regions are prone to ectopic recombination. We also assess patterns of selection by comparing dN/dS ratios of gene family members in subtelomeric versus internal regions, and we include the important antigenic gene family var. These analyses illustrate the highly dynamic nature of the karyotype of P. falciparum, and provide a method for exploring genome dynamics in other lineages.

Chromosome End Repair and Genome Stability in Plasmodium falciparum

The human malaria parasite Plasmodium falciparum replicates within circulating red blood cells, where it is subjected to conditions that frequently cause DNA damage. The repair of DNA double-stranded breaks (DSBs) is thought to rely almost exclusively on homologous recombination (HR), due to a lack of efficient nonhomologous end joining. However, given that the parasite is haploid during this stage of its life cycle, the mechanisms involved in maintaining genome stability are poorly understood. Of particular interest are the subtelomeric regions of the chromosomes, which contain the majority of the multicopy variant antigen-encoding genes responsible for virulence and disease severity. Here, we show that parasites utilize a competitive balance between de novo telomere addition, also called “telomere healing,” and HR to stabilize chromosome ends. Products of both repair pathways were observed in response to DSBs that occurred spontaneously during routine in vitro culture or resulted from experimentally induced DSBs, demonstrating that both pathways are active in repairing DSBs within subtelomeric regions and that the pathway utilized was determined by the DNA sequences immediately surrounding the break. In combination, these two repair pathways enable parasites to efficiently maintain chromosome stability while also contributing to the generation of genetic diversity.

Malaria is a major global health threat, causing approximately 430,000 deaths annually. This mosquito-transmitted disease is caused by Plasmodium parasites, with infection with the species Plasmodium falciparum being the most lethal. Mechanisms underlying DNA repair and maintenance of genome integrity in P. falciparum are not well understood and represent a gap in our understanding of how parasites survive the hostile environment of their vertebrate and insect hosts. Our work examines DNA repair in real time by using single-molecule real-time (SMRT) sequencing focused on the subtelomeric regions of the genome that harbor the multicopy gene families important for virulence and the maintenance of infection. We show that parasites utilize two competing molecular mechanisms to repair double-strand breaks, homologous recombination and de novo telomere addition, with the pathway used being determined by the surrounding DNA sequence. In combination, these two pathways balance the need to maintain genome stability with the selective advantage of generating antigenic diversity.

Saccharomyces cerevisiae as a Model to Study Replicative Senescence Triggered by Telomere Shortening

In many somatic human tissues, telomeres shorten progressively because of the DNA-end replication problem. Consequently, cells cease to proliferate and are maintained in a metabolically viable state called replicative senescence. These cells are characterized by an activation of DNA damage checkpoints stemming from eroded telomeres, which are bypassed in many cancer cells. Hence, replicative senescence has been considered one of the most potent tumor suppressor pathways. However, the mechanism through which short telomeres trigger this cellular response is far from being understood. When telomerase is removed experimentally in Saccharomyces cerevisiae, telomere shortening also results in a gradual arrest of population growth, suggesting that replicative senescence also occurs in this unicellular eukaryote. In this review, we present the key steps that have contributed to the understanding of the mechanisms underlying the establishment of replicative senescence in budding yeast. As in mammals, signals stemming from short telomeres activate the DNA damage checkpoints, suggesting that the early cellular response to the shortest telomere(s) is conserved in evolution. Yet closer analysis reveals a complex picture in which the apparent single checkpoint response may result from a variety of telomeric alterations expressed in the absence of telomerase. Accordingly, the DNA replication of eroding telomeres appears as a critical challenge for senescing budding yeast cells and the easy manipulation of S. cerevisiae is providing insights into the way short telomeres are integrated into their chromatin and nuclear environments. Finally, the loss of telomerase in budding yeast triggers a more general metabolic alteration that remains largely unexplored. Thus, telomerase-deficient S. cerevisiae cells may have more common points than anticipated with somatic cells, in which telomerase depletion is naturally programed, thus potentially inspiring investigations in mammalian cells.

Impact of chromosome ends on the biology and virulence of Plasmodium falciparum

In recent years, many studies have focused on heterochromatin located at chromosome ends, which plays an important role in regulating gene expression in many organisms ranging from yeast to humans. Similarly, in the protozoan Plasmodium falciparum, which is the most virulent human malaria parasite, the heterochromatin present in telomeres and subtelomeric regions exerts a silencing effect on the virulence gene families located therein. Studies addressing P. falciparum chromosome ends have demonstrated that these regions participate in other functions, such as the formation of the T-loop structure, the replication of telomeric regions, the regulation of telomere length and the formation of telomeric heterochromatin. In addition, telomeres are involved in anchoring chromosome ends to the nuclear periphery, thereby playing an important role in nuclear architecture and gene expression regulation. Here, we review the current understanding of chromosome ends, the proteins that bind to these regions and their impact on the biology and virulence of P. falciparum.

Anatomy and evolution of telomeric and subtelomeric regions in the human protozoan parasite Trypanosoma cruzi

BACKGROUND

The subtelomeres of many protozoa are highly enriched in genes with roles in niche adaptation. T. cruzi trypomastigotes express surface proteins from Trans-Sialidase (TS) and Dispersed Gene Family-1 (DGF-1) superfamilies which are implicated in host cell invasion. Single populations of T. cruzi may express different antigenic forms of TSs. Analysis of TS genes located at the telomeres suggests that chromosome ends could have been the sites where new TS variants were generated. The aim of this study is to characterize telomeric and subtelomeric regions of T. cruzi available in TriTrypDB and connect the sequences of telomeres to T. cruzi working draft sequence.

RESULTS

We first identified contigs carrying the telomeric repeat (TTAGGG). Of 49 contigs identified, 45 have telomeric repeats at one end, whereas in four contigs the repeats are located internally. All contigs display a conserved telomeric junction sequence adjacent to the hexamer repeats which represents a signature of T. cruzi chromosome ends. We found that 40 telomeric contigs are located on T. cruzi chromosome-sized scaffolds. In addition, we were able to map several telomeric ends to the chromosomal bands separated by pulsed-field gel electrophoresis.The subtelomeric sequence structure varies widely, mainly as a result of large differences in the relative abundance and organization of genes encoding surface proteins (TS and DGF-1), retrotransposon hot spot genes (RHS), retrotransposon elements, RNA-helicase and N-acetyltransferase genes. While the subtelomeric regions are enriched in pseudogenes, they also contain complete gene sequences matching both known and unknown expressed genes, indicating that these regions do not consist of nonfunctional DNA but are instead functional parts of the expressed genome. The size of the subtelomeric regions varies from 5 to 182 kb; the smaller of these regions could have been generated by a recent chromosome breakage and telomere healing event.

CONCLUSIONS

The lack of synteny in the subtelomeric regions suggests that genes located in these regions are subject to recombination, which increases their variability, even among homologous chromosomes. The presence of typical subtelomeric genes can increase the chance of homologous recombination mechanisms or microhomology-mediated end joining, which may use these regions for the pairing and recombination of free ends.

A global transcriptional analysis of Plasmodium falciparum malaria reveals a novel family of telomere-associated lncRNAs

BACKGROUND

Mounting evidence suggests a major role for epigenetic feedback in Plasmodium falciparum transcriptional regulation. Long non-coding RNAs (lncRNAs) have recently emerged as a new paradigm in epigenetic remodeling. We therefore set out to investigate putative roles for lncRNAs in P. falciparum transcriptional regulation.

RESULTS

We used a high-resolution DNA tiling microarray to survey transcriptional activity across 22.6% of the P. falciparum strain 3D7 genome. We identified 872 protein-coding genes and 60 putative P. falciparum lncRNAs under developmental regulation during the parasite's pathogenic human blood stage. Further characterization of lncRNA candidates led to the discovery of an intriguing family of lncRNA telomere-associated repetitive element transcripts, termed lncRNA-TARE. We have quantified lncRNA-TARE expression at 15 distinct chromosome ends and mapped putative transcriptional start and termination sites of lncRNA-TARE loci. Remarkably, we observed coordinated and stage-specific expression of lncRNA-TARE on all chromosome ends tested, and two dominant transcripts of approximately 1.5 kb and 3.1 kb transcribed towards the telomere.

CONCLUSIONS

We have characterized a family of 22 telomere-associated lncRNAs in P. falciparum. Homologous lncRNA-TARE loci are coordinately expressed after parasite DNA replication, and are poised to play an important role in P. falciparum telomere maintenance, virulence gene regulation, and potentially other processes of parasite chromosome end biology. Further study of lncRNA-TARE and other promising lncRNA candidates may provide mechanistic insight into P. falciparum transcriptional regulation.

Sirtuin mechanism and inhibition: explored with N(ε)-acetyl-lysine analogs

Silent information regulator 2 (Sir2) enzymes or sirtuins are a family of intracellular protein deacetylases that can catalyze the β-nicotinamide adenine dinucleotide (β-NAD(+))-dependent deacetylation of N(ε)-acetyl-lysine on protein substrates, with the formation of lysine N(ε)-deacetylated protein species and small molecule products, i.e. nicotinamide and 2'-O-acetyl-ADP-ribose (2'-O-AADPR). These enzymes are evolutionarily conserved among all the three kingdoms of life, with the yeast Sir2 being the founding family member. In humans, seven sirtuins, i.e. SIRT1-7, have been identified. The past a few years have witnessed a tremendous interest in investigating the unique mechanism for the sirtuin-catalyzed deacetylation reaction. We have also seen a lot of research employing different strategies to identify different types of the inhibitors for this enzymatic deacetylation reaction. These inhibitors hold great potential toward a fuller exploration of sirtuin biology and pharmacology as well as toward developing novel therapeutics for metabolic and age-related diseases and cancer. Here we would like to review the significant contributions that the judicious use of a variety of N(ε)-acetyl-lysine analogs has been able to make toward our enhanced mechanistic understanding and capability of pharmacological exploitation of the sirtuin-catalyzed deacetylation reaction.

Telomeric heterochromatin in Plasmodium falciparum

Until very recently, little was known about the chromatin structure of the telomeres and subtelomeric regions in Plasmodium falciparum. In yeast and Drosophila melanogaster, chromatin structure has long been known to be an important aspect in the regulation and functioning of these regions. Telomeres and subtelomeric regions are enriched in epigenetic marks that are specific to heterochromatin, such as methylation of lysine 9 of histone H3 and lysine 20 of histone H4. In P. falciparum, histone modifications and the presence of both the heterochromatin "writing" (PfSir2, PKMT) and "reading" (PfHP1) machinery at telomeric and subtelomeric regions indicate that these regions are likely to have heterochromatic structure that is epigenetically regulated. This structure may be important for telomere functions such as the silencing of the var gene family implicated in the cytoadherence and antigenic variation of these parasites.

Sir2 paralogues cooperate to regulate virulence genes and antigenic variation in Plasmodium falciparum

Cytoadherance of Plasmodium falciparum-infected erythrocytes in the brain, organs and peripheral microvasculature is linked to morbidity and mortality associated with severe malaria. Parasite-derived P. falciparum Erythrocyte Membrane Protein 1 (PfEMP1) molecules displayed on the erythrocyte surface are responsible for cytoadherance and undergo antigenic variation in the course of an infection. Antigenic variation of PfEMP1 is achieved by in situ switching and mutually exclusive transcription of the var gene family, a process that is controlled by epigenetic mechanisms. Here we report characterisation of the P. falciparum silent information regulator's A and B (PfSir2A and PfSir2B) and their involvement in mutual exclusion and silencing of the var gene repertoire. Analysis of P. falciparum parasites lacking either PfSir2A or PfSir2B shows that these NAD(+)-dependent histone deacetylases are required for silencing of different var gene subsets classified by their conserved promoter type. We also demonstrate that in the absence of either of these molecules mutually exclusive expression of var genes breaks down. We show that var gene silencing originates within the promoter and PfSir2 paralogues are involved in cis spreading of silenced chromatin into adjacent regions. Furthermore, parasites lacking PfSir2A but not PfSir2B have considerably longer telomeric repeats, demonstrating a role for this molecule in telomeric end protection. This work highlights the pivotal but distinct role for both PfSir2 paralogues in epigenetic silencing of P. falciparum virulence genes and the control of pathogenicity of malaria infection.

Differential association of Orc1 and Sir2 proteins to telomeric domains in Plasmodium falciparum

Telomeres have the capacity to recruit proteins that facilitate the spreading of heterochromatin into subtelomeric DNA regions. In the human protozoan pathogen Plasmodium falciparum, the telomere-associated protein Sir2 has been shown to control the silencing of members of virulence genes at some, but not all, chromosome-end loci, indicating that additional proteins are involved in telomere position effect. Here, we identified, in P. falciparum, a novel telomere-associated protein that displays homology with the origin-of-recognition-complex 1 protein Orc1. Antibodies raised against this P. falciparum protein localized to telomeric clusters in the nuclear periphery and the nucleolus. It was found that, prior to DNA replication, P. falciparum Orc1 and Sir2 undergo drastic subcellular reorganization, such as dissociation from the telomere cluster and spreading into the nucleus and parasite cytoplasm. Relocation of Orc1 and Sir2 was also linked to the partial dissociation of telomere clusters. Super gel-shift and chromatin-immunoprecipitation experiments showed the physical association of Orc1 with telomere repeats but revealed a differential association with adjacent non-coding repeat DNA elements. Our data suggest that Plasmodium telomeres might fold back and that Orc1 cooperates with Sir2 in telomeric silencing.

Antigenic variation in Plasmodium falciparum

The persistence of the human malaria parasite Plasmodium falciparum during blood stage proliferation in its host depends on the successive expression of variant molecules at the surface of infected erythrocytes. This variation is mediated by the differential control of a family of surface molecules termed PfEMP1 encoded by approximately 60 var genes. Each individual parasite expresses a single var gene at a time, maintaining all other members of the family in a transcriptionally silent state. PfEMP1/var enables parasitized erythrocytes to adhere within the microvasculature, resulting in severe disease. This review highlights key regulatory mechanisms thought to be critical for monoallelic expression of var genes. Antigenic variation is orchestrated by epigenetic factors including monoallelic var transcription at separate spatial domains at the nuclear periphery, differential histone marks on otherwise identical var genes, and var silencing mediated by telomeric heterochromatin. In addition, controversies surrounding var genetic elements in antigenic variation are discussed.

A highly conserved segmental duplication in the subtelomeres of Plasmodium falciparum chromosomes varies in copy number

Background

Segmental duplications (SD) have been found in genomes of various organisms, often accumulated at the ends of chromosomes. It has been assumed that the sequence homology in-between the SDs allow for ectopic interactions that may contribute to the emergence of new genes or gene variants through recombinatorial events.

Methods

In silico analysis of the 3D7 Plasmodium falciparum genome, conducted to investigate the subtelomeric compartments, led to the identification of subtelomeric SDs. Sequence variation and copy number polymorphisms of the SDs were studied by DNA sequencing, real-time quantitative PCR (qPCR) and fluorescent in situ hybridization (FISH). The levels of transcription and the developmental expression of copy number variant genes were investigated by qPCR.

Results

A block of six genes of >10 kilobases in size, including var, rif, pfmc-2tm and three hypothetical genes (n-, o- and q-gene), was found duplicated in the subtelomeric regions of chromosomes 1, 2, 3, 6, 7, 10 and 11 (SD1). The number of SD1 per genome was found to vary from 4 to 8 copies in between different parasites. The intragenic regions of SD1 were found to be highly conserved across ten distinct fresh and long-term cultivated P. falciparum. Sequence variation was detected in a ≈ 23 amino-acid long hypervariable region of a surface-exposed loop of PFMC-2TM. A hypothetical gene within SD1, the n-gene, encoding a PEXEL/VTS-containing two-transmembrane protein was found expressed in ring stage parasites. The n-gene transcription levels were found to correlate to the number of n-gene copies. Fragments of SD1 harbouring two or three of the SD1-genes (o-gene, pfmc-2tm, q-gene) were also found in the 3D7 genome. In addition a related second SD, SD2, of ≈ 55% sequence identity to SD1 was found duplicated in a fresh clinical isolate but was only present in a single copy in 3D7 and in other P. falciparum lines or clones.

Conclusion

Plasmodium falciparum carries multiple sequence conserved SDs in the otherwise highly variable subtelomeres of its chromosomes. The uniqueness of the SDs amongst plasmodium species, and the conserved nature of the genes within, is intriguing and suggests an important role of the SD to P. falciparum.

Sequencing and analysis of chromosome 1 of Eimeria tenella reveals a unique segmental organization

Eimeria tenella is an intracellular protozoan parasite that infects the intestinal tracts of domestic fowl and causes coccidiosis, a serious and sometimes lethal enteritis. Eimeria falls in the same phylum (Apicomplexa) as several human and animal parasites such as Cryptosporidium, Toxoplasma, and the malaria parasite, Plasmodium. Here we report the sequencing and analysis of the first chromosome of E. tenella, a chromosome believed to carry loci associated with drug resistance and known to differ between virulent and attenuated strains of the parasite. The chromosome--which appears to be representative of the genome--is gene-dense and rich in simple-sequence repeats, many of which appear to give rise to repetitive amino acid tracts in the predicted proteins. Most striking is the segmentation of the chromosome into repeat-rich regions peppered with transposon-like elements and telomere-like repeats, alternating with repeat-free regions. Predicted genes differ in character between the two types of segment, and the repeat-rich regions appear to be associated with strain-to-strain variation.

A sirtuin in the African trypanosome is involved in both DNA repair and telomeric gene silencing but is not required for antigenic variation

Silent information regulator 2 (Sir2)-related proteins or sirtuins function as NAD(+)-dependent deacetylases or ADP ribosylases that target a range of substrates, thereby influencing chromatin structure and a diverse range of other biological functions. Genes encoding three Sir2-related proteins (SIR2rp1-3) have been identified in the parasitic trypanosomatids, early branching protozoa with no previously reported transcriptional silencing machinery. Here we show that, in the mammalian-infective bloodstream-stage of the African trypanosome, Trypanosoma brucei, SIR2rp1 localizes to the nucleus while SIR2rp2 and SIR2rp3 are both mitochondrial proteins. The nuclear protein, SIR2rp1, controls DNA repair and repression of RNA polymerase I-mediated expression immediately adjacent to telomeres. Antigenic variation, however, which involves the silencing and Pol I-mediated transcriptional switching of subtelomeric variant surface glycoprotein genes, continues to operate independent of SIR2rp1.

Proof of interaction between Leishmania SIR2RP1 deacetylase and chaperone HSP83

The cytoplasmic Leishmania silent information regulator 2 (SIR2)RP1 protein is essential for parasite growth and survival and constitutes an attractive therapeutic target. Little information is available on putative substrate(s) and/or partner(s) that could shed light on the pathways in which this enzyme plays a role. We carried out co-immunoprecipitation experiments on the soluble fractions of wild-type and parasites overexpressing LmSIR2RP1 and found that the essential chaperone heat shock protein (HSP) 83, the Leishmania ortholog of the mammalian HSP90, constantly co-immunoprecipitated with LmSIR2RP1. We found that Leishmania HSP83 is among the lysine acetylated protein, but the intracellular level of SIR2RP1 does not influence the acetylation status of HSP83. Finally, the modified Geldanamycin susceptibility (an inhibitor of HSP83) exhibited by SIR2RP1 mutant parasites support an in vivo relationship between the chaperone activity of HSP83 and LmSIR2RP1. An insight on the nature of the interaction in Leishmania is required to understand its role in the cell fate control during cytodifferentiation.

Evidence that Plasmodium falciparum chromosome end clusters are cross-linked by protein and are the sites of both virulence gene silencing and activation

The malaria parasite Plasmodium falciparum undergoes antigenic variation through allelic exclusion and variant expression of surface proteins encoded by the var gene family. Regulation of var genes is under epigenetic control and involves reversible silencing and activation that requires the physical repositioning of a var locus into a transcriptionally permissive zone of the nuclear periphery. P. falciparum chromosome ends appear to aggregate into large perinuclear clusters which house both subtelomeric and chromosome central var genes. In this study we further define the composition of telomeric clusters using fluorescent in situ hybridization, and provide evidence that chromosome end clusters are formed by cross-linking protein. In addition, we demonstrate that a subtelomeric reporter gene and a var gene remain within clusters regardless of their transcriptional status. Our findings support a model whereby a highly localized structure dedicated to the activation of a single var gene can be housed within a gene dense chromosome end cluster that is otherwise transcriptionally silent.