The Molecular Mechanisms of Giardia Encystation

Protists: Eukaryotic single-celled organisms and the functioning of their organelles

Organelles are membrane bound structures that compartmentalize biochemical and molecular functions. With improved molecular, biochemical and microscopy tools the diversity and function of protistan organelles has increased in recent years, providing a complex panoply of structure/function relationships. This is particularly noticeable with the description of hydrogenosomes, and the diverse array of structures that followed, having hybrid hydrogenosome/mitochondria attributes. These diverse organelles have lost the major, at one time, definitive components of the mitochondrion (tricarboxylic cycle enzymes and cytochromes), however they all contain the machinery for the assembly of Fe-S clusters, which is the single unifying feature they share. The plasticity of organelles, like the mitochondrion, is therefore evident from its ability to lose its identity as an aerobic energy generating powerhouse while retaining key ancestral functions common to both aerobes and anaerobes. It is interesting to note that the apicoplast, a non-photosynthetic plastid that is present in all apicomplexan protozoa, apart from Cryptosporidium and possibly the gregarines, is also the site of Fe-S cluster assembly proteins. It turns out that in Cryptosporidium proteins involved in Fe-S cluster biosynthesis are localized in the mitochondrial remnant organelle termed the mitosome. Hence, different organisms have solved the same problem of packaging a life-requiring set of reactions in different ways, using different ancestral organelles, discarding what is not needed and keeping what is essential. Don't judge an organelle by its cover, more by the things it does, and always be prepared for surprises.

A Dual Omics Approach to Evaluate Transcriptional and Metabolic Responses During Lipid Deprivation in an Oyster Parasite, Perkinsus marinus

Perkinsus marinus, a protozoan and the causative agent of perkinsosis (dermo disease) is a prevalent parasite found within the eastern oyster (Crassostrea virginica). In this study, we explore metabolic processes of P. marinus cells under lipid-depleted medium conditions to elucidate the interchanging flux of lipid and carbohydrate metabolism. Although P. marinus can synthesize their own lipids from available nutrients, they display a slower growth in medium not supplemented with lipids as opposed to medium with lipids. Under these conditions, using transcriptomics, we surprisingly observed evidence of stimulated lipid degradation through increased transcription of two core β-oxidation pathway enzymes. Simultaneously, phospholipid biosynthetic pathways were downregulated. Metabolomic analysis supported the transcriptomic results. Most fatty acids were decreased in lipid-deplete medium as opposed to lipid-replete medium, and available glucose was fermented to lactate. A significant increase in the cholesterol derivative zymosterol further supported a downregulation of membrane synthesis under the experimental conditions. A robust tricarboxylic acid (TCA) cycle was apparent by enhanced citrate synthase transcription, and a simultaneous reduction in branched chain amino acids. It is concluded that although P. marinus has the capacity for synthesizing its own lipids, it can respond to lipid deprivation in medium by oxidizing readily available stores, and likely transitioning into a resting stage.

Giardia Colonizes and Encysts in High-Density Foci in the Murine Small Intestine

Giardia is a single-celled parasite causing significant diarrheal disease in several hundred million people worldwide. Due to limited access to the site of infection in the gastrointestinal tract, our understanding of the dynamics of Giardia infections in the host has remained limited and largely inferred from laboratory culture. To better understand Giardia physiology and colonization in the host, we developed imaging methods to quantify Giardia expressing bioluminescent physiological reporters in two relevant animal models. We discovered that parasites primarily colonize and encyst in the proximal small intestine in discrete, high-density foci. We also show that high parasite density contributes to encystation initiation.

Giardia lamblia is a highly prevalent yet understudied protistan parasite causing significant diarrheal disease worldwide. Hosts ingest Giardia cysts from contaminated sources. In the gastrointestinal tract, cysts excyst to become motile trophozoites, colonizing and attaching to the gut epithelium. Trophozoites later differentiate into infectious cysts that are excreted and contaminate the environment. Due to the limited accessibility of the gut, the temporospatial dynamics of giardiasis in the host are largely inferred from laboratory culture and thus may not mirror Giardia physiology in the host. Here, we have developed bioluminescent imaging (BLI) to directly interrogate and quantify the in vivo temporospatial dynamics of Giardia infection, thereby providing an improved murine model to evaluate anti-Giardia drugs. Using BLI, we determined that parasites primarily colonize the proximal small intestine nonuniformly in high-density foci. By imaging encystation-specific bioreporters, we show that encystation initiates shortly after inoculation and continues throughout the duration of infection. Encystation also initiates in high-density foci in the proximal small intestine, and high density contributes to the initiation of encystation in laboratory culture. We suggest that these high-density in vivo foci of colonizing and encysting Giardia likely result in localized disruption to the epithelium. This more accurate visualization of giardiasis redefines the dynamics of the in vivo Giardia life cycle, paving the way for future mechanistic studies of density-dependent parasitic processes in the host.

IMPORTANCEGiardia is a single-celled parasite causing significant diarrheal disease in several hundred million people worldwide. Due to limited access to the site of infection in the gastrointestinal tract, our understanding of the dynamics of Giardia infections in the host has remained limited and largely inferred from laboratory culture. To better understand Giardia physiology and colonization in the host, we developed imaging methods to quantify Giardia expressing bioluminescent physiological reporters in two relevant animal models. We discovered that parasites primarily colonize and encyst in the proximal small intestine in discrete, high-density foci. We also show that high parasite density contributes to encystation initiation.

Transcriptomic Profiling of High-Density Giardia Foci Encysting in the Murine Proximal Intestine

Giardia is a highly prevalent, understudied protistan parasite causing significant diarrheal disease worldwide. Its life cycle consists of two stages: infectious cysts ingested from contaminated food or water sources, and motile trophozoites that colonize and attach to the gut epithelium, later encysting to form new cysts that are excreted into the environment. Current understanding of parasite physiology in the host is largely inferred from transcriptomic studies using Giardia grown axenically or in co-culture with mammalian cell lines. The dearth of information about the diversity of host-parasite interactions occurring within distinct regions of the gastrointestinal tract has been exacerbated by a lack of methods to directly and non-invasively interrogate disease progression and parasite physiology in live animal hosts. By visualizing Giardia infections in the mouse gastrointestinal tract using bioluminescent imaging (BLI) of tagged parasites, we recently showed that parasites colonize the gut in high-density foci. Encystation is initiated in these foci throughout the entire course of infection, yet how the physiology of parasites within high-density foci in the host gut differs from that of cells in laboratory culture is unclear. Here we use BLI to precisely select parasite samples from high-density foci in the proximal intestine to interrogate in vivo Giardia gene expression in the host. Relative to axenic culture, we noted significantly higher expression (>10-fold) of oxidative stress, membrane transporter, and metabolic and structural genes associated with encystation in the high-density foci. These differences in gene expression within parasite foci in the host may reflect physiological changes associated with high-density growth in localized regions of the gut. We also identified and verified six novel cyst-specific proteins, including new components of the cyst wall that were highly expressed in these foci. Our in vivo transcriptome data support an emerging view that parasites encyst early in localized regions in the gut, possibly as a consequence of nutrient limitation, and also impact local metabolism and physiology.

Aquatic biomonitoring of Giardia cysts and Cryptosporidium oocysts in peninsular Malaysia

An aquatic biomonitoring of Giardia cysts and Cryptosporidium oocysts in river water corresponding to five villages situated in three states in peninsular Malaysia was determined. There were 51.3% (20/39) and 23.1% (9/39) samples positive for Giardia and Cryptosporidium (oo)cysts, respectively. Overall mean concentration between villages for Giardia cysts ranged from 0.10 to 25.80 cysts/l whilst Cryptosporidium oocysts ranged from 0.10 to 0.90 oocysts/l. Detailed results of the river samples from five villages indicated that Kuala Pangsun 100% (9/9), Kemensah 77.8% (7/9), Pos Piah 33.3% (3/9) and Paya Lebar 33.3% (1/3) were contaminated with Giardia cysts whilst Cryptosporidium (oo)cysts were only detected in Kemensah (100 %; 9/9) and Kuala Pangsun (66.6%; 6/9). However, the water samples from Bentong were all negative for these waterborne parasites. Samples were collected from lower point, midpoint and upper point. Midpoint refers to the section of the river where the studied communities are highly populated. Meanwhile, the position of the lower point is at least 2 km southward of the midpoint and upper point is at least 2 km northward of the midpoint. The highest mean concentration for (oo)cysts was found at the lower points [3.15 ± 6.09 (oo)cysts/l], followed by midpoints [0.66 ± 1.10 (oo)cysts/l] and upper points [0.66 ± 0.92 (oo)cysts/l]. The mean concentration of Giardia cysts was highest at Kuala Pangsun (i.e. 5.97 ± 7.0 cysts/l), followed by Kemensah (0.83 ± 0.81 cysts/l), Pos Piah (0.20 ± 0.35 cysts/l) and Paya Lebar (0.10 ± 0.19 cysts/l). On the other hand, the mean concentration of Cryptosporidium oocysts was higher at Kemensah (0.31 ± 0.19 cysts/l) compared to Kuala Pangsun (0.03 ± 0.03cysts/l). All the physical and chemical parameters did not show significant correlation with both protozoa. In future, viability status and molecular characterisation of Giardia and Cryptosporidium should be applied to identify species and genotypes/subgenotypes for better understanding of the epidemiology of these waterborne parasites.

The ubiquitin-activating enzyme (E1) of the early-branching eukaryote Giardia intestinalis shows unusual proteolytic modifications and play important roles during encystation

Giardia intestinalis is considered an early-branching eukaryote and is therefore a valuable model for studying primordial cellular processes. This work reports the characterization of the ubiquitin-activating enzyme (E1) during growth and different stages of trophozoite differentiation into cysts. We found that in Giardia E1 expression (both at mRNA and protein levels) is regulated during encystation. The enzyme is proteolytically processed mainly into two fragments of 68kDa (N-terminal) and 47kDa (C-terminal). This phenomenon has not been described for any other E1. In trophozoites, this enzyme localized at spots within the cytoplasm as detected by using polyclonal antibodies against either E1 N- or C-terminal fragments. This pattern changed during encystation into a diffuse localization throughout the cytoplasm of encysting cells. E1 localizes in mature cysts at cytoplasmic spots and in the cyst wall. Our antisense silencing experiments suggested that E1 is an essential gene for parasite viability. On the other hand, E1 over-expression greatly increased the encystation rate, indicating a relationship between E1 and Giardia differentiation.

Rab11 and actin cytoskeleton participate in Giardia lamblia encystation, guiding the specific vesicles to the cyst wall

Background

Giardia passes through two stages during its life cycle, the trophozoite and the cyst. Cyst formation involves the synthesis of cyst wall proteins (CWPs) and the transport of CWPs into encystation-specific vesicles (ESVs). Active vesicular trafficking is essential for encystation, but the molecular machinery driving vesicular trafficking remains unknown. The Rab proteins are involved in the targeting of vesicles to several intracellular compartments through their association with cytoskeletal motor proteins.

Methodology and Principal Findings

In this study, we found a relationship between Rab11 and the actin cytoskeleton in CWP1 transport. Confocal microscopy showed Rab11 was distributed throughout the entire trophozoite, while in cysts it was translocated to the periphery of the cell, where it colocalized with ESVs and microfilaments. Encystation was also accompanied by changes in rab11 mRNA expression. To evaluate the role of microfilaments in encystation, the cells were treated with latrunculin A. Scanning electron microscopy showed this treatment resulted in morphological damages to encysted parasites. The intensity of fluorescence-labeled Rab11 and CWP1 in ESVs and cyst walls was reduced, and rab11 and cwp1 mRNA levels were down-regulated. Furthermore, knocking down Rab11 with a hammerhead ribozyme resulted in an up to 80% down-regulation of rab11 mRNA. Although this knockdown did not appear lethal for trophozoites and did not affect cwp1 expression during the encystation, confocal images showed CWP1 was redistributed throughout the cytosol.

Conclusions and Significance

Our results indicate that Rab11 participates in the early and late encystation stages by regulating CWP1 localization and the actin-mediated transport of ESVs towards the periphery. In addition, alterations in the dynamics of actin affected rab11 and cwp1 expression. Our results provide new information about the molecules involved in Giardia encystation and suggest that Rab11 and actin may be useful as novel pharmacological targets.

The encystation process is crucial for survival and transmission of Giardia lamblia to new hosts. During this process, vesicular trafficking and the cytoskeleton play important roles. In eukaryotic cells, intracellular transport is regulated by proteins, including Rab-GTPases and SNAREs, which regulate vesicle formation along with recognition of and binding to the target membrane. Cytoskeletal structures are also involved in these processes. In this study, we demonstrate the participation of Rab11 in the transport of encystation-specific vesicles (ESVs). Additionally, we demonstrate that disruption of actin microfilaments affects ESVs transport. The modification of actin dynamics was also correlated with a reduction in rab11 and cwp1 expression. Furthermore, down-regulation of rab11 mRNA by a specific hammerhead ribozyme caused nonspecific localization of CWP1. We thus provide new information about the molecular machinery that regulates Giardia lamblia encystation. Given our findings, Rab11 and actin may be useful targets to block Giardia encystation.

Epigenetic mechanisms regulate stage differentiation in the minimized protozoan Giardia lamblia

Histone modification is an important mechanism regulating both gene expression and the establishment and maintenance of cellular phenotypes during development. Regulation of histone acetylation via histone acetylases and deacetylases (HDACs) appears to be particularly crucial in determining gene expression patterns. In this study we explored the effect of HDAC inhibition on the life cycle of the human pathogen Giardia lamblia, a highly reduced parasitic protozoan characterized by minimized cellular processes. We found that the HDAC inhibitor FR235222 increased the level of histone acetylation and induced transcriptional regulation of approximately 2% of genes in proliferating and encysting parasites. In addition, our analyses showed that the levels of histone acetylation decreased during differentiation into cysts, the infective stage of the parasite. Importantly, FR235222 treatment during encystation reversed this histone hypo-acetylation and potently blocked the formation of cysts. These results provide the first direct evidence for epigenetic regulation of gene expression in this simple eukaryote. This suggests that regulation of histone acetylation is involved in the control of Giardia stage differentiation, and identifies epigenetic mechanisms as a promising target to prevent Giardia transmission.

Participation of actin on Giardia lamblia growth and encystation

Background

Microfilaments play a determinant role in different cell processes such as: motility, cell division, phagocytosis and intracellular transport; however, these structures are poorly understood in the parasite Giardia lamblia.

Methodology and Principal Findings

By confocal microscopy using TRITC-phalloidin, we found structured actin distributed in the entire trophozoite, the label stand out at the ventral disc, median body, flagella and around the nuclei. During Giardia encystation, a sequence of morphological changes concurrent to modifications on the distribution of structured actin and in the expression of actin mRNA were observed. To elucidate whether actin participates actively on growth and encystation, cells were treated with Cytochalasin D, Latrunculin A and Jasplakinolide and analyzed by confocal and scanning electron microscopy. All drugs caused a growth reduction (27 to 45%) and changes on the distribution of actin. Besides, 60 to 80% of trophozoites treated with the drugs, exhibited damage at the caudal region, alterations in the flagella and wrinkles-like on the plasma membrane. The drugs also altered the cyst-yield and the morphology, scanning electron microscopy revealed diminished cytokinesis, cysts with damages in the wall and alterations in the size and on the intermembranal space. Furthermore, the drugs caused a significant reduction of the intensity of flourescence-labeled CWP1 on ESV and on cyst wall, this was coincident with a reduction of CWP1 gene expression (34%).

Conclusions and Significance

All our results, indicated an important role of actin in the morphology, growth and encystation and indirectly suggested an actin role in gene expression.

Characterization of SNAREs determines the absence of a typical Golgi apparatus in the ancient eukaryote Giardia lamblia

Giardia is a eukaryotic protozoal parasite with unusual characteristics, such as the absence of a morphologically evident Golgi apparatus. Although both constitutive and regulated pathways for protein secretion are evident in Giardia, little is known about the mechanisms involved in vesicular docking and fusion. In higher eukaryotes, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) of the vesicle-associated membrane protein and syntaxin families play essential roles in these processes. In this work we identified and characterized genes for 17 SNAREs in Giardia to define the minimal set of subcellular organelles present during growth and encystation, in particular the presence or not of a Golgi apparatus. Expression and localization of all Giardia SNAREs demonstrate their presence in distinct subcellular compartments, which may represent the extent of the endomembrane system in eukaryotes. Remarkably, Giardia SNAREs, homologous to Golgi SNAREs from other organisms, do not allow the detection of a typical Golgi apparatus in either proliferating or differentiating trophozoites. However, some features of the Golgi, such as the packaging and sorting function, seem to be performed by the endoplasmic reticulum and/or the nuclear envelope. Moreover, depletion of individual genes demonstrated that several SNAREs are essential for viability, whereas others are dispensable. Thus, Giardia requires a smaller number of SNAREs compared with other eukaryotes to accomplish all of the vesicle trafficking events that are critical for the growth and differentiation of this important human pathogen.

Transcription of metabolic enzyme genes during the excystation of Giardia lamblia

The present study evaluates the expression of genes of Giardia lamblia, one of the most simple and most early diverging eukaryotes, that encode the metabolic enzymes pyruvate: ferredoxin oxidoreductase (PFOR), acetyl-CoA synthetase (ACS), alcohol dehydrogenase E (ADHE) and glutamate dehydrogenase (GDH) and the cyst wall protein (CWP1) gene in trophozoites, cysts and during the excystation process. Primers were designed to amplify mRNA fragments through quantitative reverse-transcriptase-polymerase-chain-reaction. In trophozoites, all transcripts of the enzymes studied were present. In cysts, three of the transcripts were detected: CWP1, GDH and ACS; but the relative levels of the mRNA of GDH and ACS were very different between trophozoites and cysts. During excystation, PFOR and ADHE transcripts appeared after the first induction phase, and the mRNAs of ACS and GDH increased throughout the process.

Potential involvement of extracellular signal-regulated kinase 1 and 2 in encystation of a primitive eukaryote, Giardia lamblia. Stage-specific activation and intracellular localization

Mitogen-activated protein kinase (MAPK) pathways are major signaling systems by which eukaryotic cells convert environmental cues to intracellular events such as proliferation and differentiation. We have identified Giardia lamblia homologues of two members of the MAPK family ERK1 and ERK2. Functional characterization of giardial ERK1 and ERK2 revealed that both kinases were expressed in trophozoites and encysting cells as 44- and 41-kDa polypeptides, respectively, and were catalytically active. Analysis of the kinetic parameters of the recombinant proteins showed that ERK2 is approximately 5 times more efficient than ERK1 in phosphorylating myelin basic protein as a substrate, although the phosphorylating efficiency of the native ERK1 and ERK2 appeared to be the same. Immunofluorescence analysis of the subcellular localization of ERK1 and ERK2 in trophozoites showed ERK1 staining mostly in the median body and in the outer edges of the adhesive disc and ERK2 staining in the nuclei and in the caudal flagella. Our study also showed a noticeable change in the subcellular distribution of ERK2 during encystation, which became more punctate and mostly cytoplasmic, but no significant change in the ERK1 localization at any time during encystation. Interestingly, both ERK1 and ERK2 enzymes exhibited a significantly reduced kinase activity during encystation reaching a minimum at 24 h, except for an initial approximately 2.5-fold increase in the ERK1 activity at 2 h, which resumed back to the normal levels at 48 h despite no apparent change in the expression level of either one of these kinases in encysting cells. A reduced concentration of the phosphorylated ERK1 and ERK2 was also evident in these cells at 24 h. Our study suggests a functional distinction between ERK1 and ERK2 and that these kinases may play a critical role in trophozoite differentiation into cysts.

Identification and characterization of a novel secretory granule calcium-binding protein from the early branching eukaryote Giardia lamblia

Giardia lamblia is a flagellate protozoan that infects humans and other mammals and the most frequently isolated intestinal parasite worldwide. Giardia trophozoites undergo essential biological changes to survive outside the intestine of their host by differentiating into infective cysts. Cyst formation, or encystation, is considered one of the most primitive adaptive responses developed by eukaryotes early in evolution and crucial for the transmission of the parasite among susceptible hosts. During this process, proteins that will assemble into the extracellular cyst wall (CWP1 and CWP2) are transported to the cell surface within encystation-specific secretory vesicles (ESVs) by a developmentally regulated secretory pathway. Cyst wall proteins (CWPs) are maintained as a dense material inside the ESVs, but after exocytosis, they form the fibrillar matrix of the cyst wall. Little is known about the molecular mechanisms involved in granule biogenesis and discharge in Giardia, as well as the assembly of the extracellular wall. In this work, we provide evidences that a novel 54-kDa protein that exclusively localizes to the ESVs is induced during encystation similar to CWPs, proteolytically processed during granule maturation, and able to bind calcium in vitro. The gene encoding this molecule predicts a novel protein (called gGSP for G. lamblia Granule-specific Protein) without homology to any other protein reported in public databases. Nevertheless, it possesses characteristics of calcium-sequestering molecules of higher eukaryotes. Inhibition of gGSP expression abolishes cyst wall formation, suggesting that this secretory granule protein regulates Ca(2+)-dependent degranulation of ESVs during cyst wall formation.

Membrane-associated dipeptidyl peptidase IV is involved in encystation-specific gene expression during Giardia differentiation

Giardia is a flagellated protozoan that resides in the upper small intestine of its vertebrate host and is the most common cause of defined waterborne diarrhoea worldwide. Giardia trophozoites undergo significant biological changes to survive outside the host by differentiating into infective cysts. Encystation is thus essential for transmission of the parasite among susceptible hosts. In the present study, we report that bestatin, a competitive inhibitor of aminopeptidases, blocks cyst formation in vitro by abolishing the expression of encystation-specific genes, such as those coding for cyst wall proteins. Bestatin does not affect proliferating trophozoites, indicating that its effect is encystation-specific. Using biochemical and molecular biological approaches, we identified the enzyme inhibited by bestatin and cloned its corresponding gene. Sequence similarity indicated that this enzyme belongs to a family of dipeptidyl peptidases. Our results suggest that a specific proteolytic event caused by a constitutively expressed membrane-associated dipeptidyl peptidase IV is necessary for encystation of Giardia.

The activity of a developmentally regulated cysteine proteinase is required for cyst wall formation in the primitive eukaryote Giardia lamblia

Giardia is an intestinal parasite that belongs to the earliest diverging branch of the eukaryotic lineage of descent. Giardia undergoes adaptation for survival outside the host's intestine by differentiating into infective cysts. Encystation involves the synthesis and transport of cyst wall constituents to the plasma membrane for release and extracellular organization. Nevertheless, little is known about the molecular events related to cyst wall biogenesis in Giardia. Among the components of the cyst wall there are two proteins that we have previously identified and characterized: CWP1 (26 kDa) and CWP2 (39 kDa). Expression of these proteins is coordinately induced, and both concentrated within encystation-specific secretory vesicles before their extracellular polymerization. Although highly similar to each other at the amino terminus, CWP2 includes a COOH-terminal 121-amino acid extension. Here, we show that this extension, rich in basic residues, is cleaved from CWP2 before cyst wall formation by an intracellular cysteine proteinase activity, which is induced during encystation like CWPs. Specific inhibitors prevent release of cyst wall materials, abolishing cyst wall formation. We also report the purification, cloning, and characterization of the encystation-specific cysteine proteinase responsible for the proteolytic processing of CWP2, which is homologue to lysosomal cathepsin C. Encystation-specific cysteine proteinase ESCP possesses unique characteristics compared with cathepsins from higher eukaryotes, such as a transmembrane domain and a short cytoplasmic tail. These features make this enzyme the most divergent cathepsin C identified to date and provide new insights regarding cyst wall formation in Giardia.

Chromatin and histones from Giardia lamblia: a new puzzle in primitive eukaryotes

The three deepest eukaryote lineages in small subunit ribosomal RNA phylogenies are the amitochondriate Microsporidia, Metamonada, and Parabasalia. They are followed by either the Euglenozoa (e.g., Euglena and Trypanosoma) or the Percolozoa as the first mitochondria-containing eukaryotes. Considering the great divergence of histone proteins in protozoa we have extended our studies of histones from Trypanosomes (Trypanosoma cruzi, Crithidia fasciculata and Leishmania mexicana) to the Metamonada Giardia lamblia, since Giardia is thought to be one of the most primitive eukaryotes. In the present work, the structure of G. lamblia chromatin and the histone content of the soluble chromatin were investigated and compared with that of higher eukaryotes, represented by calf thymus. The chromatin is present as nucleosome filaments which resemble the calf thymus array in that they show a more regular arrangement than those described for Trypanosoma. SDS-polyacrylamide gel electrophoresis and protein characterization revealed that the four core histones described in Giardia are in the same range of divergence with the histones from other lower eukaryotes. In addition, G. lamblia presented an H1 histone with electrophoretic mobility resembling the H1 of higher eukaryotes, in spite of the fact that H1 has a different molecular mass in calf thymus. Giardia also presents a basic protein which was identified as an HU-like DNA-binding protein usually present in eubacteria, indicating a chimaeric composition for the DNA-binding protein set in this species. Finally, the phylogenetic analysis of selected core histone protein sequences place Giardia divergence before Trypanosoma, despite the fact that Trypanosoma branch shows an acceleration in the evolutionary rate pointing to an unusual evolutionary behavior in this lineage.