Gene cloning, expression, and enzyme kinetics analysis of Eimeria tenella 2- methylcitrate synthase

In prokaryotes and lower eukaryotes, 2-methylcitrate cycle (2-MCC) is the main pathway for propionate decomposition and transformation, but little is known about the 2-MCC pathway of Eimeria tenella. The analysis of genomic data found that the coding gene of 2- methylcitrate synthase (EC 2.3.3.5, PrpC) exists in E. tenella, which is a key enzyme of 2-MCC pathway. Through the search analysis of the database (ToxoDB), it was found that ETH_ 00026655 contains the complete putative sequence of EtprpC. In this study, we amplified the ORF sequence of EtprpC based on putative sequence. Then, prokaryotic expression, enzyme activity and kinetic analysis was performed. The results showed that the EtprpC ORF sequence was 1272 bp, encoding a 46.3 kDa protein comprising 424 amino acids. Enzyme activity assays demonstrate linearity between the initial reaction rate (OD/min) and EtPrpC concentration (ranging from 1.5 to 9 µg/reaction), with optimal enzyme activity observed at 41°C and pH 8.0. The results of enzymatic kinetic analysis showed that the Km of EtPrpC for propionyl-CoA, oxaloacetic acid, and acetyl-CoA was 5.239 ± 0.17 mM, 1.102 ± 0.08 μM, and 5.999 ± 1.24 μM, respectively. The Vmax was 191.11 ± 19.1 nmol/min/mg, 225.48 ± 14.4 nmol/min/mg, and 370.02 ± 25.8 nmol/min/mg when EtPrpC concentration at 4, 6, and 8 μg, respectively. Although the ability of EtPrpC to catalyze acetyl-CoA is only 0.11% of its ability to catalyze propionyl-CoA, it indicates that the 2-MCC pathway in E. tenella is similar to that in bacteria and may have a bypass function in the TCA cycle. This study can provide the theoretical foundation for the new drug targets and the development of new anticoccidial drugs.

Structural and Biochemical Features of Eimeria tenella Dihydroorotate Dehydrogenase, a Potential Drug Target

Dihydroorotate dehydrogenase (DHODH) is a mitochondrial monotopic membrane protein that plays an essential role in the pyrimidine de novo biosynthesis and electron transport chain pathways. In Eimeria tenella, an intracellular apicomplexan parasite that causes the most severe form of chicken coccidiosis, the activity of pyrimidine salvage pathway at the intracellular stage is negligible and it relies on the pyrimidine de novo biosynthesis pathway. Therefore, the enzymes of the de novo pathway are considered potential drug target candidates for the design of compounds with activity against this parasite. Although, DHODHs from E. tenella (EtDHODH), Plasmodium falciparum (PfDHODH), and human (HsDHODH) show distinct sensitivities to classical DHODH inhibitors, in this paper, we identify ferulenol as a potent inhibitor of both EtDHODH and HsDHODH. Additionally, we report the crystal structures of EtDHODH and HsDHODH in the absence and presence of ferulenol. Comparison of these enzymes showed that despite similar overall structures, the EtDHODH has a long insertion in the N-terminal helix region that assumes a disordered configuration. In addition, the crystal structures revealed that the ferulenol binding pocket of EtDHODH is larger than that of HsDHODH. These differences can be explored to accelerate structure-based design of inhibitors specifically targeting EtDHODH.

Cloning and characterization of three Eimeria tenella lipid phosphate phosphatases

Although lipid phosphate phosphatases (LPPs) play an important role in cellular signaling in addition to lipid biosynthesis, little is thus far known about parasite LPPs. In this study, we characterized three Eimeria tenella cDNA clones encoding LPP named EtLPP1, EtLPP2 and EtLPP3. Key structural features previously described in LPPs, including the three conserved domains proposed as catalytic sites, a single conserved N-glycosylation site, and putative transmembrane domains were discovered in the three resulting EtLPP amino acid sequences. Expression of His6-tagged EtLPP1, -2, and -3 in HEK293 cells produced immunoreactive proteins with variable molecular sizes, suggesting the presence of multiple forms of each of the three EtLPPs. The two faster-migrating protein bands below each of the three EtLPP proteins were found to be very similar to the porcine 35-kDa LPP enzyme in their molecular size and the extent of their N-glycosylation, suggesting that the three EtLPPs are partially N-glycosylated. Kinetic analyses of the activity of the three enzymes against PA, LPA, C1P and S1P showed that Km values for each of the substrates were (in μM) 284, 46, 28, and 22 for EtLPP1; 369, 179, 237, and 52 for EtLPP2; and 355, 83, and 260 for EtLPP3. However, EtLPP3 showed negligible activity on S1P. These results confirmed that the three EtLPPs have broad substrate specificity. The results also indicated that despite structural similarities, the three EtLPPs may play distinct functions through their different models of substrate preference. Furthermore, particularly high expression levels of the three EtLPP genes were detected in the sporozoite stage of the E. tenella life cycle (p<0.001), suggesting that their encoded proteins might play an important biological function in the sporozoite stage.

Eimeripain, a cathepsin B-like cysteine protease, expressed throughout sporulation of the apicomplexan parasite Eimeria tenella

The invasion and replication of Eimeria tenella in the chicken intestine is responsible for avian coccidiosis, a disease that has major economic impacts on poultry industries worldwide. E. tenella is transmitted to naïve animals via shed unsporulated oocysts that need contact with air and humidity to form the infectious sporulated oocysts, which contain the first invasive form of the parasite, the sporozoite. Cysteine proteases (CPs) are major virulence factors expressed by protozoa. In this study, we show that E. tenella expresses five transcriptionally regulated genes encoding one cathepsin L, one cathepsin B and three cathepsin Cs. Biot-LC-LVG-CHN₂, a cystatin derived probe, tagged eight polypeptides in unsporulated oocysts but only one in sporulated oocysts. CP-dependant activities were found against the fluorescent substrates, Z-FR-AMC and Z-LR-AMC, throughout the sporulation process. These activities corresponded to a cathepsin B-like enzyme since they were inhibited by CA-074, a specific cathepsin B inhibitor. A 3D model of the catalytic domain of the cathepsin B-like protease, based on its sequence homology with human cathepsin B, further confirmed its classification as a papain-like protease with similar characteristics to toxopain-1 from the related apicomplexan parasite, Toxoplasma gondii; we have, therefore, named the E. tenella cathepsin B, eimeripain. Following stable transfection of E. tenella sporozoites with a plasmid allowing the expression of eimeripain fused to the fluorescent protein mCherry, we demonstrated that eimeripain is detected throughout sporulation and has a punctate distribution in the bodies of extra- and intracellular parasites. Furthermore, CA-074 Me, the membrane-permeable derivative of CA-074, impairs invasion of epithelial MDBK cells by E. tenella sporozoites. This study represents the first characterization of CPs expressed by a parasite from the Eimeria genus. Moreover, it emphasizes the role of CPs in transmission and dissemination of exogenous stages of apicomplexan parasites.

Identification and partial characterization of a serine protease inhibitor (serpin) of Eimeria tenella

Serine protease inhibitors (serpins) mediate many biological processes, including immune responses to pathogenic infection. In this study, a member of the serpin superfamily was identified from the common poultry parasite Eimeria tenella by expressed sequence tag analysis and the rapid amplification of cDNA ends technique. The full-length cDNA was 1,918 bp and had an open reading frame of 1,248 bp encoding a polypeptide of 415 amino acids with the theoretical isoelectric point of 5.26 and predicted molecular weight of 45.5 kDa. Real-time quantitative PCR analysis revealed that the serpin gene was expressed at higher levels in sporozoites than in the other developmental stages (unsporulated oocysts, sporulated oocysts, and second-generation merozoites). The sequence encoding the mature protein was amplified by PCR, cloned into the pET28(a) vector, and expressed in Escherichia coli. Specific antiserum generated against the recombinant protein was prepared and used to determine invasion inhibition capacity and localization; the results suggested that the serpin may play an important role in invasion and survival of the sporoziotes in the host.

Type I and type II fatty acid biosynthesis in Eimeria tenella: enoyl reductase activity and structure

Apicomplexan parasites of the genus Eimeria are the major causative agent of avian coccidiosis, leading to high economic losses in the poultry industry. Recent results show that Eimeria tenella harbours an apicoplast organelle, and that a key biosynthetic enzyme, enoyl reductase, is located in this organelle. In related parasites, enoyl reductase is one component of a type II fatty acid synthase (FAS) and has proven to be an attractive target for antimicrobial compounds. We cloned and expressed the mature form of E. tenella enoyl reductase (EtENR) for biochemical and structural studies. Recombinant EtENR exhibits NADH-dependent enoyl reductase activity and is inhibited by triclosan with an IC50 value of 60 nm. The crystal structure of EtENR reveals overall similarity with other ENR enzymes; however, the active site of EtENR is unoccupied, a state rarely observed in other ENR structures. Furthermore, the position of the central beta-sheet appears to block NADH binding and would require significant movement to allow NADH binding, a feature not previously seen in the ENR family. We analysed the E. tenella genomic database for orthologues of well-characterized bacterial and apicomplexan FAS enzymes and identified 6 additional genes, suggesting that E. tenella contains a type II FAS capable of synthesizing saturated, but not unsaturated, fatty acids. Interestingly, we also identified sequences that appear to encode multifunctional type I FAS enzymes, a feature also observed in Toxoplasma gondii, highlighting the similarity between these apicomplexan parasites.

Serine protease activity in developmental stages of Eimeria tenella

A number of complex processes are involved in Eimeria spp. survival, including control of sporulation, intracellular invasion, evasion of host immune responses, successful reproduction, and nutrition. Proteases have been implicated in many of these processes, but the occurrence and functions of serine proteases have not been characterized. Bioinformatic analysis suggests that the Eimeria tenella genome contains several serine proteases that lack homology to trypsin. Using RT-PCR, a gene encoding a subtilisin-like and a rhomboid protease-like serine protease was shown to be developmentally regulated, both being poorly expressed in sporozoites (SZ) and merozoites (MZ). Casein substrate gel electrophoresis of oocyst extracts during sporulation demonstrated bands of proteolytic activity with relative molecular weights (Mr) of 18, 25, and 45 kDa that were eliminated by coincubation with serine protease inhibitors. A protease with Mr of 25 kDa was purified from extracts of unsporulated oocysts by a combination of affinity and anion exchange chromatography. Extracts of SZ contained only a single band of inhibitor-sensitive proteolytic activity at 25 kDa, while the pattern of proteases from extracts of MZ was similar to that of oocysts except for the occurrence of a 90 kDa protease, resistant to protease inhibitors. Excretory-secretory products (ESP) from MZ contained AEBSF (4-[2-Aminoethyl] benzenesulphonyl fluoride)-sensitive protease activity with a specific activity about 10 times greater than that observed in MZ extracts. No protease activity was observed in the ESP from SZ. Pretreatment of SZ with AEBSF significantly reduced SZ invasion and the release of the microneme protein, MIC2. The current results suggest that serine proteases are present in all the developmental stages examined.

Biochemical characterization of enoyl reductase involved in Type II fatty acid synthesis in the intestinal coccidium Eimeria tenella (Phylum Apicomplexa)

An enoyl reductase (EtENR) closely related to those of green algae and involved in Type II fatty acid synthesis was characterized and localized to the apicoplast in the coccidium Eimeria tenella. Biochemical analysis using native EtENR protein extracted from parasites confirmed its function as an enoyl reductase using NADH as a cofactor. However, the recombinant form (rEtENR) expressed in bacteria was only able to oxidize NADH, but unable to transfer the electron to enoyl-CoA, possibly due to the inappropriate folding of rEtENR expressed in bacteria. The functions of both native and recombinant EtENR could be inhibited by triclosan (IC(50)=1.45 microM), suggesting that this enzyme may be explored as a drug target against coccidiosis.

Influence of monensin on cation influx and Na+ -K+ -ATPase activity of Eimeria tenella sporozoites in vitro

The experiments were conducted to determine intrasporozoite Na+/K+ concentrations (by AAS) and membrane-bound Na+ -K+ -ATPase activity (measured by UV-VIS with a Na+ -K+ -ATPase Detection Kit) of Eimeria tenella sporozoites of the sensitive line (i.e., the parent line, coded as OS) and 2 resistant lines, derived from the parent line (coded as OR125 and OR200), with and without in vitro exposure to monensin. These parameters for OR125 and OR200 were significantly lower than those for OS. In vitro exposure to monensin increased intrasporozoite Na+/K+ concentrations and Na+ -K+ -ATPase activity, but the stimulation on OS was significantly higher than those on OR125 and OR200, indicating that monensin had less effect on resistant parasites. The results of this study suggest that altered biochemical or physiological properties, or both, in the membranes of E. tenella might be related to a reduced sensitivity to monensin.

Eimeria tenella enolase and pyruvate kinase: A likely role in glycolysis and in others functions

Two cDNA codings for glycolytic enzymes were cloned from a cDNA library constructed from the schizont stage of the avian parasite Eimeria tenella. Enolase and pyruvate kinase cDNA were fully sequenced and compared with sequences of enzymes from other organisms. Although these enzymes were already detected in the sporozoite stage, their expression was enhanced during the first schizogony in accordance with the anaerobic conditions of this part of the life cycle of the parasite. Under activating conditions, microscopic observations suggest that these glycolytic enzymes were relocalised inside sporozoites and moreover were in part secreted. The enzymes were also localised at the apex of the first generation of merozoites. Enolase was partly observed inside the nucleus of sporozoites and schizonts. Taken together, these results suggest that glycolytic enzymes not only have a function in glycolysis during anaerobic intracellular stages but may also participate in the invasion process and, for enolase, in the control of gene regulation.

Anticoccidial kinase inhibitors: Identification of protein kinase targets secondary to cGMP-dependent protein kinase

Trisubstituted pyrrole inhibitors of the essential coccidian parasite cGMP dependent protein kinase (PKG) block parasite invasion and show in vivo efficacy against Eimeria in chickens and Toxoplasma in mice. An imidazopyridine inhibitor of PKG activity with greater potency in both parasite invasion assays and in vivo activity has recently been identified. Susceptibility experiments with a Toxoplasma knock-out strain expressing a complementing compound-refractory PKG allele ('T761Q-KO'), suggest a role for additional secondary protein kinase targets. Using extracts from this engineered T. gondii strain and a radiolabeled imidazopyridine ligand, a single peak of binding activity associated with calmodulin-like domain protein kinase (CDPK1) has been identified. Like PKG, CDPK1 has been implicated in host cell invasion and exhibits sub-nanomolar sensitivity to the compound. Amino acid sequence comparisons of coccidian CDPKs and a mutational analysis reveal that the binding of the ligand to PKG and CDPK1 (but not other CDPK isoforms) is mediated by similar contacts in a catalytic site hydrophobic binding pocket, and can be blocked by analogous amino acid substitutions. Transgenic strains over-expressing a biochemically active but compound-refractory CDPK1 mutant ('G128Q') fail to show reduced susceptibility to the compound in vivo, suggesting that selective inhibition of this enzyme is not responsible for the enhanced anti-parasitic potency of the imidazopyridine analog. An alternative secondary target candidate, the alpha-isoform of casein kinase 1 (CK1alpha), shows sensitivity to the compound in the low nanomolar range. These results provide an example of the utility of the Toxoplasma model system for investigating the mechanism of action of novel anticoccidial agents.

Synthesis and SAR Studies of diarylpyrrole anticoccidial agents

2-(4-Fluorophenyl)-3-(4-pyridinyl)-5-substituted pyrroles were prepared and evaluated as anticoccidial agents in both in vitro and in vivo assays. Among the compounds evaluated, the dimethylamine-substituted pyrrole 19a is the most potent inhibitor of Eimeria tenella PKG (cGMP-dependent protein kinase). Further SAR studies on the side chain of the 2-pyrrolidine nitrogen did not enhance in vivo anticoccidial activity.

Partial purification and characterization of an aminopeptidase from Eimeria tenella

Our previous investigation demonstrated the expression in Eimeria tenella sporulated oocysts of an aminopeptidase (AP) with strong homology to AP N. To further understand the role of proteases during development, we investigated the molecular and biochemical properties of E. tenella AP. Greater than 95% AP activity was present in a soluble extract during sporulation of oocysts with highest activity in fully sporulated oocysts. The AP activity was inhibited by the AP inhibitors bestatin and 1,6-phenanthroline, but not by serine protease inhibitors. The AP had specificity for synthetic endopeptidase substrates that contain arginine, alanine, or glycine at the N terminus. Partial purification of the enzyme yielded a major protein band with an Mr of about 106 kDa and an isoelectric point (Ip) of 5.1. Reverse transcription-polymerase chain reaction indicated that the gene for AP is expressed during sporulation, but expression is absent or greatly reduced in the sporozoites and merozoites. On the basis of the deduced gene structure, the predicted Mr is 110 kDa with a pI of 5.59. Database search indicates that the E. tenella AP shares significant homology with the AP from Apicomplexan taxa: Toxoplasma gondii, Cryptosporidium parvum, and Cryptosporidium hominis. Together, these results confirm the presence of a cytosolic AP related to AP N, which is expressed and active during sporulation of E. tenella oocysts.

Synthesis and SAR studies of very potent imidazopyridine antiprotozoal agents

Compounds 10a (IC50 110 pM) and 21 (IC50 40 pM) are the most potent inhibitors of Eimeria tenella cGMP-dependent protein kinase activity reported to date and are efficacious in the in vivo antiparasitic assay when administered to chickens at 12.5 and 6.25 ppm levels in the feed. However, both compounds are positive in the Ames microbial mutagenesis assay which precludes them from further development as antiprotozoal agents in the absence of negative lifetime rodent carcinogenicity studies.

Eimeria tenella: identification of secretory and surface proteins from expressed sequence tags

To identify new vaccine candidates, Eimeria tenella expressed sequence tags (ESTs) from public databases were analysed for secretory molecules with an especially developed automated in silico strategy termed DNAsignalP. A total of 12,187 ESTs were clustered into 2881 contigs followed by a blastx search, which resulted in a significant number of E. tenella contigs with homologies to entries in public databases. Amino acid sequences of appropriate homologous proteins were analysed for the occurrence of an N-terminal signal sequence using the algorithm signalP. The resulting list of 84 entries comprised 51 contigs whose deduced proteins showed homologies to proteins of apicomplexan parasites. Based on function or localisation, we selected candidate proteins classified as (i) secreted proteins of Apicomplexa parasites, (ii) secreted enzymes, and (iii) transport and signalling proteins. To verify our strategy experimentally, we used a functional complementation system in yeast. For five selected candidate proteins we found that these were indeed secreted. Our approach thus represents an efficient method to identify secretory and surface proteins out of EST databases.

Overproduction of heterologous mannitol 1-phosphatase: a key factor for engineering mannitol production by Lactococcus lactis

To achieve high mannitol production by Lactococcus lactis, the mannitol 1-phosphatase gene of Eimeria tenella and the mannitol 1-phosphate dehydrogenase gene mtlD of Lactobacillus plantarum were cloned in the nisin-dependent L. lactis NICE overexpression system. As predicted by a kinetic L. lactis glycolysis model, increase in mannitol 1-phosphate dehydrogenase and mannitol 1-phosphatase activities resulted in increased mannitol production. Overexpression of both genes in growing cells resulted in glucose-mannitol conversions of 11, 21, and 27% by the L. lactis parental strain, a strain with reduced phosphofructokinase activity, and a lactate dehydrogenase-deficient strain, respectively. Improved induction conditions and increased substrate concentrations resulted in an even higher glucose-to-mannitol conversion of 50% by the lactate dehydrogenase-deficient L. lactis strain, close to the theoretical mannitol yield of 67%. Moreover, a clear correlation between mannitol 1-phosphatase activity and mannitol production was shown, demonstrating the usefulness of this metabolic engineering approach.

metaSHARK: software for automated metabolic network prediction from DNA sequence and its application to the genomes of Plasmodium falciparum and Eimeria tenella

The metabolic SearcH And Reconstruction Kit (metaSHARK) is a new fully automated software package for the detection of enzyme-encoding genes within unannotated genome data and their visualization in the context of the surrounding metabolic network. The gene detection package (SHARKhunt) runs on a Linux system and requires only a set of raw DNA sequences (genomic, expressed sequence tag and/or genome survey sequence) as input. Its output may be uploaded to our web-based visualization tool (SHARKview) for exploring and comparing data from different organisms. We first demonstrate the utility of the software by comparing its results for the raw Plasmodium falciparum genome with the manual annotations available at the PlasmoDB and PlasmoCyc websites. We then apply SHARKhunt to the unannotated genome sequences of the coccidian parasite Eimeria tenella and observe that, at an E-value cut-off of 10(-20), our software makes 142 additional assertions of enzymatic function compared with a recent annotation package working with translated open reading frame sequences. The ability of the software to cope with low levels of sequence coverage is investigated by analyzing assemblies of the E.tenella genome at estimated coverages from 0.5x to 7.5x. Lastly, as an example of how metaSHARK can be used to evaluate the genomic evidence for specific metabolic pathways, we present a study of coenzyme A biosynthesis in P.falciparum and E.tenella.

AnEimeriavaccine candidate appears to be lactate dehydrogenase; characterization and comparative analysis

AnEimeria acervulinaprotein fraction was identified which conferred partial protection against anE. acervulinachallenge infection. From this fraction a 37 kDa protein was purified and its corresponding cDNA was cloned and shown to encode a lactate dehydrogenase (LDH). Full length cDNAs encoding LDH from two related species,E. tenellaandE. maxima, were also cloned. The homology between the primary amino acid sequences of these threeEimeriaLDH enzymes was rather low (66–80%), demonstrating an evolutionary divergence. ThePlasmodiumLDH crystal structure was used to generate a 3D-model structure ofE. tenellaLDH, which demonstrated that the many variations in the primary amino acid sequences (P. falciparumLDH andE. tenellaLDH show only 47% identity) had not resulted in altered 3D-structures. Only a single LDH gene was identified inEimeria, which was active as a homotetramer. The protein was present at similar levels throughout different parasitic stages (oocysts, sporozoites, schizonts and merozoites), but its corresponding RNA was only observed in the schizont stage, suggesting that its synthesis is restricted to the intracellular stage.

A role for coccidian cGMP-dependent protein kinase in motility and invasion

The coccidian parasite cGMP-dependent protein kinase is the primary target of a novel coccidiostat, the trisubstituted pyrrole 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H-pyrrol-3-yl] pyridine (compound 1), which effectively controls the proliferation of Eimeria tenella and Toxoplasma gondii parasites in animal models. The efficacy of compound 1 in parasite-specific metabolic assays of infected host cell monolayers is critically dependent on the timing of compound addition. Simultaneous addition of compound with extracellular E. tenella sporozoites or T. gondii tachyzoites inhibited [3H]-uracil uptake in a dose-dependent manner, while minimal efficacy was observed if compound addition was delayed, suggesting a block in host cell invasion. Immunofluorescence assays confirmed that compound 1 blocks the attachment of Eimeria sporozoites or Toxoplasma tachyzoites to host cells and inhibits parasite invasion and gliding motility. Compound 1 also inhibits the secretion of micronemal adhesins (E. tenella MIC1, MIC2 and T. gondii MIC2), an activity closely linked to invasion and motility in apicomplexan parasites. The inhibition of T. gondii MIC2 adhesin secretion by compound 1 was not reversed by treatment with calcium ionophores or by ethanol (a microneme secretagogue), suggesting a block downstream of calcium-dependent events commonly associated with the discharge of the microneme organelle in tachyzoites. Transgenic Toxoplasma strains expressing cGMP-dependent protein kinase mutant alleles that are refractory to compound 1 (including cGMP-dependent protein kinase knock-out lines complemented by such mutants) were used as tools to validate the potential role of cGMP-dependent protein kinase in invasion and motility. In these strains, parasite adhesin secretion, gliding motility, host cell attachment and invasion displayed a reduced sensitivity to compound 1. These data clearly demonstrate that cGMP-dependent protein kinase performs an important role in the host-parasite interaction.

Toxoplasma gondii cyclic GMP-dependent kinase: chemotherapeutic targeting of an essential parasite protein kinase

The trisubstituted pyrrole 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H-pyrrol-3-yl]pyridine (compound 1) has in vivo activity against the apicomplexan parasites Toxoplasma gondii and Eimeria tenella in animal models. The presumptive molecular target of this compound in E. tenella is cyclic GMP-dependent protein kinase (PKG). Native PKG purified from T. gondii has kinetic and pharmacologic properties similar to those of the E. tenella homologue, and both have been functionally expressed as recombinant proteins in T. gondii. Computer modeling of parasite PKG was used to predict catalytic site amino acid residues that interact with compound 1. The recombinant laboratory-generated mutants T. gondii PKG T761Q or T761M and the analogous E. tenella T770 alleles have reduced binding affinity for, and are not inhibited by, compound 1. By all other criteria, PKG with this class of catalytic site substitution is indistinguishable from wild-type enzyme. A genetic disruption of T. gondii PKG can only be achieved if a complementing copy of PKG is provided in trans, arguing that PKG is an essential protein. Strains of T. gondii, disrupted at the genomic PKG locus and dependent upon the T. gondii T761-substituted PKGs, are as virulent as wild type in mice. However, unlike mice infected with wild-type T. gondii that are cured by compound 1, mice infected with the laboratory-generated strains of T. gondii do not respond to treatment. We conclude that PKG represents the primary molecular target responsible for the antiparasitic efficacy of compound 1.

Purification and molecular characterization of cGMP-dependent protein kinase from Apicomplexan parasites. A novel chemotherapeutic target

The trisubstituted pyrrole 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H-pyrrol-3-yl]pyridine (Compound 1) inhibits the growth of Eimeria spp. both in vitro and in vivo. The molecular target of Compound 1 was identified as cGMP-dependent protein kinase (PKG) using a tritiated analogue to purify a approximately 120-kDa protein from lysates of Eimeria tenella. This represents the first example of a protozoal PKG. Cloning of PKG from several Apicomplexan parasites has identified a parasite signature sequence of nearly 300 amino acids that is not found in mammalian or Drosophila PKG and which contains an additional, third cGMP-binding site. Nucleotide cofactor regulation of parasite PKG is remarkably different from mammalian enzymes. The activity of both native and recombinant E. tenella PKG is stimulated 1000-fold by cGMP, with significant cooperativity. Two isoforms of the parasite enzyme are expressed from a single copy gene. NH(2)-terminal sequence of the soluble isoform of PKG is consistent with alternative translation initiation within the open reading frame of the enzyme. A larger, membrane-associated isoform corresponds to the deduced full-length protein sequence. Compound 1 is a potent inhibitor of both soluble and membrane-associated isoforms of native PKG, as well as recombinant enzyme, with an IC(50) of <1 nm.

Molecular characterization of a coccidian parasite cGMP dependent protein kinase

The cGMP-dependent protein kinase (PKG) of Eimeria tenella and Toxoplasma gondii is the target of a novel coccidiostat that is effective against coccidiosis and toxoplasmosis in animal models. Preparations of native PKG enzyme from Toxoplasma and Eimeria contain a membrane-associated polypeptide (isoform-I) of about 110 kDa and a slightly smaller soluble polypeptide (isoform-II). Expression of T. gondii and E. tenella PKG cDNA clones in Toxoplasma yield similarly sized recombinant polypeptides, which co-migrate on SDS-polyacrylamide gels with the corresponding native isoforms. Results of targeted mutagenesis of potential translational initiation sites suggest that parasite isoform-II is a product of alternative translational initiation from an internal initiator methionine codon. Exclusive expression of isoform-II or isoform-I can be achieved by preventing initiation at the respective primary or secondary sites. Immunofluorescence analysis indicates that recombinant isoform-I localizes primarily to the parasite plasma membrane, while isoform-II remains cytosolic. Mutagenesis and metabolic labeling studies reveal that the observed membrane-association of full-length recombinant PKG is mediated by N-terminal myristoylation and palmitoylation at amino acids G2 and C4. We also confirm the functional significance of a putative third PKG allosteric site, common to apicomplexan PKGs but absent from vertebrate or insect PKGs. In assays with transiently transfected parasites, constructs harboring a mutation at this site express markedly lower levels of cGMP-dependent PKG activity, while a triple mutant bearing mutations in all three sites reduces kinase activity to background levels.

The role of a parasite-specific allosteric site in the distinctive activation behavior of Eimeria tenella cGMP-dependent protein kinase

A cGMP-dependent protein kinase (PKG) was recently identified as an anticoccidial target for the apicomplexan parasite Eimeria tenella [Gurnett, A., Liberator, P. A., Dulski, P., Salowe, S., Donald, R. G. K., Anderson, J., Wiltsie, J., Diaz, C., Harris, G., Chang, B., Darkin-Rattray, S. J., Nare, B., Crumley, T., Blum, P., Misura, A., Tamas, T., Sardana, M., Yuan, J., Biftu, T., and Schmatz, D. (2002) J. Biol. Chem. (in press)]. Unlike the PKGs of higher organisms that have two cGMP binding sites in their regulatory domain, the PKG from Eimeria tenella (Et-PKG) contains three putative cGMP binding sites and has distinctive activation properties, including a very large stimulation by cGMP ( approximately 1000-fold) with significant cooperativity (Hill coefficient of 1.7). During our investigation of Et-PKG activation, we found that 8-substituted cGMP analogues are weak partial activators. For example, 8-NBD-cGMP provides a maximal stimulation of activity of only 20-fold with little evident cooperativity, although cGMP can synergize with the analogue to provide full activation. The results suggest that partial activation is a consequence of restricted binding of 8-NBD-cGMP to a subset of cGMP sites in the enzyme. Site-directed mutagenesis of conserved arginine and glutamate residues in the parasite-specific third cGMP site confirms that this site is an important functional participant in the allosteric regulation of the kinase and that it exhibits very high selectivity against 8-NBD-cGMP. Since the results are consistent with full activation of Et-PKG requiring cyclic nucleotide binding in all three allosteric sites, one role for the additional cGMP site may be to establish a stricter regulatory mechanism for the kinase activity than is present in the PKGs of higher organisms containing only two allosteric sites.

Molecular characterization and expression of a putative ATPase domain from Eimeria tenella

VCP (Valosin-Containing Protein), a member of the AAA (ATPases Associated to a variety of cellular Activities) family of proteins, possesses a duplicated highly conserved ATPase domain. An expressed sequence tag (EST), representing a clone from the Eimeria tenella merozoite cDNA library, was found to have high similarity to VCP genes from other organisms. A complete sequence derived from the corresponding clone (designated eth060) shows amino acid identity of 42-62% with other members of the VCP subfamily. Sequence analysis identified a putative ATPase domain in the eth060 sequence. This domain was PCR-amplified using gene-specific primers and cloned into a pBAD/Thio-TOPO expression vector. Expression in Escherichia coli demonstrated that the putative ATPase domain, which consists of 414 amino acid residues, produced a fusion protein of approximately 60 kDa in size.

Nitrophenide (Megasul) blocks Eimeria tenella development by inhibiting the mannitol cycle enzyme mannitol-1-phosphate dehydrogenase

Unsporulated oocysts of the protozoan parasite Eimeria tenella contain high levels of mannitol, which is thought to be the principal energy source for the process of sporulation. Biosynthesis and utilization of this sugar alcohol occurs via a metabolic pathway known as the mannitol cycle. Here, results are presented that suggest that 3-nitrophenyl disulfide (nitrophenide, Megasul), an anticoccidial drug commercially used in the 1950s, inhibits mannitol-1-phosphate dehydrogenase (M1PDH), which catalyzes the committed enzymatic step in the mannitol cycle. Treatment of E. tenella-infected chickens with nitrophenide resulted in a 90% reduction in oocyst shedding. The remaining oocysts displayed significant morphological abnormalities and were largely incapable of further development. Nitrophenide treatment did not affect parasite asexual reproduction, suggesting specificity for the sexual stage of the life cycle. Isolated oocysts from chickens treated with nitrophenide exhibited a dose-dependent reduction in mannitol, suggesting in vivo inhibition of parasite mannitol biosynthesis. Nitrophenide-mediated inhibition of MIPDH was observed in vitro using purified native enzyme. Moreover, MIPDH activity immunoprecipitated from E. tenella-infected cecal tissues was significantly lower in nitrophenide-treated compared with untreated chickens. Western blot analysis and immunohistochemistry showed that parasites from nitrophenide-treated and untreated chickens contained similar enzyme levels. These data suggest that nitrophenide blocks parasite development at the sexual stages by targeting M1PDH. Thus, targeting of the mannitol cycle with drugs could provide an avenue for controlling the spread of E. tenella in commercial production facilities by preventing oocyst shedding.

Aspartyl proteinase genes from apicomplexan parasites: evidence for evolution of the gene structure

Aspartyl proteinases are a widely distributed family of enzymes. All vertebrate aspartyl proteinases share a conserved nine-exon gene structure, but in other organisms the structure of aspartyl proteinase genes varies considerably. The exon-intron patterns generally reflect phylogeny based on amino acid sequences. However, close comparison of these gene structures reveals some striking features, such as the conservation of intron positions and intron phases between aspartyl proteinases from nematodes and apicomplexans. Here, we discuss the implications of gene structure for the possible evolution of the aspartyl proteinase family, with particular reference to the plasmepsins of Plasmodium falciparum and eimepsin from Eimeria tenella.

Differential localisation of an Eimeria tenella aspartyl proteinase during the infection process

Aspartyl proteinases are essential for the survival of many pathogens. A single copy gene in species of Eimeria encodes an aspartyl proteinase, which we propose should be called eimepsin to conform to the commonly used names of this family of proteinases. An epitope map, constructed using BIAcore technology, confirmed the specificity of 14 mAbs for eimepsin and defined four antigenic domains, which were conserved between native and recombinant forms of eimepsin. In resting sporozoites, mAb defining antigenic domains I and II stained the refractile body organelles, whereas those defining antigenic domains III and IV stained cytoplasmic granules. During host cell invasion, the staining patterns of mAb defining antigenic domains I, III and IV changed dramatically with the apical tips of invading sporozoites becoming strongly stained. In contrast, mAb defining antigenic domain II continued to stain only the refractile bodies. During early schizogony, mAb to all four domains stained the single fused refractile body, but when schizonts matured, mAb to antigenic domains I, III and IV stained the apical tip of merozoites whereas those to antigenic domain II continued to follow the developmental redistribution of the refractile body. Irrespective of localisation, mAb to three antigenic domains recognised a polypeptide of 49 kDa, which from N-terminal sequencing corresponds to a mature form of eimepsin. Staining with fluorescent pepstatin localised a mature, active form of eimepsin to the refractile bodies of the sporozoite, schizont and first generation merozoite. It remains to be determined whether eimepsin has a catalytic function within the refractile body or whether the activated enzyme is stored in the refractile body so that it can be rapidly redistributed to the apical tip during parasite invasion.

Three enzymes newly identified from the genus Eimeria and two more newly identified from E. maxima, leading to the discovery of some aliphatic acids with activity against coccidia of the domesticated fowl

Nine enzymes were detected in sporulated oocysts of Eimeria tenella and E. maxima, parasites of the domesticated fowl (Gallus gallus). Three enzymes, hydroxybutyrate dehydrogenase, alanine aminotransferase and gamma-glutamyltransferase, all identified for the first time in Eimeria of fowl, occurred both in E. tenella and in E. maxima. The remaining enzymes assayed had previously been found in various Eimeria species of fowl, although creatine kinase and glutamate dehydrogenase were hitherto unknown from E. maxima. The three enzymes newly recorded from Eimeria of fowl are of interest as potential genetic markers, and also as potential chemotherapeutic targets. The discovery of hydroxybutyrate dehydrogenase led to the demonstration of anticoccidial activity by some aliphatic acids. The paper also includes a list of the enzymes detected in Eimeria of fowl in previous studies.

Biosynthesis and catabolism of mannitol is developmentally regulated in the protozoan parasite Eimeria tenella

The mannitol cycle is a metabolic branch of the glycolytic pathway found in Eimeria tenella. In this paper, we describe the biosynthesis and consumption of mannitol during parasite development. Low micromolar levels of mannitol were detected in all of the asexual stages and mannitol production increased sharply during the sexual phase of the life cycle. Unsporulated oocysts had high mannitol content (300 mM or 25% of the oocyst mass). Mannitol-1-phosphate dehydrogenase (M1PDH), the first committed step of the mannitol cycle, was also elevated in sexual stages and this coincides with mannitol levels. Approximately 90% of the mannitol present in unsporulated oocysts was consumed in the first 15 hr of sporulation, and levels continued to drop until the sporulation process was complete at approximately 35 hr. Thus, mannitol appears to be the "fuel" for sporulation during the vegetative stage of the parasite life cycle. Evaluation of oocyst extracts from 6 additional Eimeria species for mannitol content and the presence of M1PDH indicated that the mannitol cycle was broadly present in this genus. This finding combined with the lack of mannitol metabolism in higher eukaryotes makes this pathway an attractive chemotherapeutic target.

Molecular cloning and functional expression of mannitol-1-phosphatase from the apicomplexan parasite Eimeria tenella

A metabolic pathway responsible for the biosynthesis and utilization of mannitol is present in the seven species of Eimeria that infect chickens, but is not in the avian host. Mannitol-1-phosphatase (M1Pase), a key enzyme for mannitol biosynthesis, is a highly substrate-specific phosphatase and, accordingly, represents an attractive chemotherapeutic target. Amino acid sequence of tryptic peptides obtained from biochemically purified Eimeria tenella M1Pase was used to synthesize degenerate oligonucleotide hybridization probes. Using these reagents, a partial genomic clone and full-length cDNA clones have been isolated and characterized. The deduced amino acid sequence of E. tenella M1Pase shows limited overall homology to members of the phosphohistidine family of phosphatases. This limited homology to other histidine phosphatases does, however, include several conserved residues that have been shown to be essential for their catalytic activity. Kinetic parameters of recombinant M1Pase expressed in bacteria are essentially identical to those of the biochemically purified preparation from E. tenella. Moreover, recombinant M1Pase is subject to active site-directed, hydroxylamine-reversible inhibition by the histidine-selective acylating reagent diethyl pyrocarbonate. These results indicate the presence of an essential histidine residue(s) at the M1Pase active site, as predicted for a histidine phosphatase.

Sequence, expression and localization of calmodulin-domain protein kinases in Eimeria tenella and Eimeria maxima

We have isolated and sequenced cDNA clones from Eimeria tenella and Eimeria maxima which encode proteins that share homology with a recently described family of calmodulin-domain protein kinases. The primary sequence data show that each of the protein kinases can be divided into 2 main functional domains-an amino-terminal catalytic domain typical of serine/threonine protein kinases and a carboxy-terminal domain homologous to calmodulin, which is capable of binding calcium ions at 4 'EF-hand' motifs. Expression of the E. tenella calmodulin-domain protein kinase (EtCDPK) increased towards the end of oocyst sporulation, as judged by Northern and Western blotting, and indirect immunofluorescent antibody labelling showed that within a few minutes of adding sporozoites to target host cells in in vitro culture EtCDPK was found to be specifically associated with a filament-like structure that converges at the apical end of the parasite. Once the parasite entered the host cell EtCDPK appeared to be left on the host cell membrane at the point of entry, indicating a brief yet specific role for this molecule in the invasion of host cells by E. tenella.

Comparison of the phosphofructokinase and pyruvate kinase activities of Cryptosporidium parvum, Eimeria tenella and Toxoplasma gondii

Oocysts of Cryptosporidium parvum were shown to contain a pyrophosphate-dependent phosphofructokinase (PPi-PFK) similar to those previously described for Eimeria tenella and Toxoplasma gondii. PPi-PFK of C. parvum displayed simple hyperbolic kinetics with respect to its substrate fructose 6-phosphate and was not affected by fructose 2,6-bisphosphate, the major allosteric activator of most ATP-PFKs. Inorganic pyrophosphatase was not detectable in any of the three parasites. T. gondii tachyzoites and C. parvum cysts both contained a pyruvate kinase (PK) specific for ADP rather than PPi/AMP. The PK of T. gondii was similar to that of E. tenella in that it displayed strong positive cooperativity with respect to its substrate phosphoenolpyruvate and was heterotropically activated by glucose 6-phosphate, fructose 6-phosphate and fructose 1,6-bisphosphate. PK of C. parvum showed no evidence of allosteric properties. The results suggest that the three coccidia are similar in depending heavily on anaerobic energy production via glycolysis but that the mechanisms for regulating glycolysis are not common to all species.

Purification and characterisation of a serine-type protease from Eimeria tenella oocysts

Homogenates of sporulated oocysts of E. tenella have detectable proteolytic activity which is completely inhibited by phenylmethylsulfonyl fluoride and L-trans-epoxysuccinyl-leucyl-amido-(4-guanidino)-butane, indicating the presence of both serine and cysteine-type proteases in sporulated oocysts. A serine-type protease has been purified from the homogenate using immobilised-bacitracin affinity chromatography. The monomeric enzyme had an apparent M(r) of 20,000 and a pI of 8.6. The maximum proteolytic activity with azocasein and gelatin was observed at pH 8.0. Antibodies raised in rabbits and chickens against purified protease recognised this protein on blots of sporozoite homogenates.

Identification, partial purification and inhibition by guanine analogues of a novel enzymic activity which phosphorylates guanosine to GMP in the protozoan parasite Eimeria tenella

From oocysts of the protozoan parasite Eimeria tenella, responsible for avian coccidiosis, we have partially purified and characterized a novel enzymic activity which specifically phosphorylates guanosine to GMP. The enzyme is able to use several phosphate donors, in the order: acetyl phosphate (Ac-P) > ATP > UTP > CTP > phosphoribosyl pyrophosphate (PRPP) > dUTP > or = dATP. The low specificity of this enzyme for the phosphate donor suggested that it be named guanosine phosphotransferase (GPTase). This enzyme is biochemically distinct from the previously described adenosine kinase (AK) and hypoxanthine/xanthine/guanine phosphoribosyltransferase (HXGPRTase), and may enable the parasite to synthesize guanine nucleotides under conditions of imbalance between adenine and guanine nucleotides. Because of its possible role in the purine salvage pathways, we have studied the effect of several guanine and guanosine analogues, recently synthesized in our laboratory, on the activity of GPTase in vitro. GPTase is specifically inhibited in the micromolar range by several substituted N2-phenylguanine bases. These results indicate that, as previously found for AK and HXGPRTase, GPTase could be a potential target for antiparasitic chemotherapy.

Eimeria tenella contains a pyrophosphate-dependent phosphofructokinase and a pyruvate kinase with unusual allosteric regulators

Sporozoites and unsporulated oocysts of Eimeria tenella were shown to contain a pyrophosphate-dependent phosphofructokinase (PPi-PFK) but apparently lack an ATP-specific activity. The PPi-PFK resembles those that occur in a number of other protists in being reversible and not subject to metabolic control. In contrast, the ADP-utilising pyruvate kinase, present in two developmental stages of the parasite, exhibited strong positive cooperativity with respect to its substrate, phosphoenolpyruvate, and was shown to be allosterically activated by glucose 6-phosphate, fructose 6-phosphate and AMP. It is suggested that the PPi-PFK represents an adaptation of the parasite towards life in an environment containing only low concentrations of oxygen and that the unusual allosteric regulation of pyruvate kinase evolved to compensate for glycolysis not being controlled at the PPi-PFK step.

Amylopectin synthase of Eimeria tenella: identification and kinetic characterization

A soluble enzyme amylopectin synthase (UDP-glucose-alpha 1,4-glucan alpha-4-glucosyltransferase) which transfers glucose from uridine 5'-diphosphate glucose (UDP-glucose) to a primer to form alpha-1,4-glucosyl linkages has been identified in the extracts of unsporulated oocysts of Eimeria tenella. UDP-glucose and not ADP-glucose was the most active glucosyl donor. Corn amylopectin, rabbit liver glycogen, oyster glycogen and corn starch served as primers; the latter two were less efficient. The enzyme has an apparent pH optimum of 7.5 and exhibited typical Michaelis-Menten kinetics with dependence on both the primer and substrate concentrations. The Michaelis constants (Km), with respect to UDP-glucose, was 0.5 mM; and 0.25 mg/ml and 1.25 mg/ml with respect to amylopectin and rabbit liver glycogen. The product formed by the reaction was predominantly a glucan containing alpha-1,4 linkages. The specificity of the enzyme suggests that this enzyme is similar to glycogen synthase in eukaryotes and has been designated as amylopectin synthase (UDP-glucose-alpha-1,4-glucosetransferase EC 2.4.1.11).

Analysis of dinucleotide frequency and codon usage in the phylum Apicomplexa

Dinucleotide frequency (DiF) and codon usage (cu) were analysed in gene sequences from four parasitic protozoa, Babesia bovis, Theileria parva, Toxoplasma gondii and Eimeria tenella, of the phylum Apicomplexa. In keeping with the 'genome hypothesis', cu was found to be non-random and species specific in these organisms, although cu among members of the same subclass was found to be very similar. Several low-usage (lu) codons were identified, and the usage of lu codons appears to be related to the taxonomic position of the organisms under study. A comparison of the observed/expected DiF ratios obtained from gene coding regions revealed a low frequency of the TA and CG dinucleotides in all organisms studied. A comparison of these DiF ratios with those found in rRNA-encoding genes and in introns, showed that in the parasites, B. bovis and Th. parva (representing the piroplasms), the low frequency of dinucleotides appeared to be the result of coding pressure alone. In T. gondii and E. tenella (representing the coccidia), however, coding pressure could not completely explain differences in DiF.

Identification of a developmentally regulated sialidase in Eimeria tenella that is immunologically related to the Trypanosoma cruzi enzyme

Sporozoites and merozoites of three species of Eimeria, E. tenella, E. maxima, and E. necatrix, that cause diarrhea in chickens worldwide, were examined for their expression of sialidase (SA) activity. The enzyme was found in three species, and the activity of merozoites was 10-20 times higher than that of sporozoites. The enzyme was resistant to degradation by proteases that are normally present in the intestine, a site inhabited by the Eimeria parasites, and it was relatively resistant to heat, with optimum activity being at 40 degrees C, which is within the range of temperature in the chicken intestine (40-43 degrees C). E. tenella SA was immunoprecipitated by monoclonal and polyclonal antibodies raised against the Trypanosoma cruzi SA (TCSA), and enzyme activity was neutralized by these antibodies. E. tenella SA was identified by immunoblots as a doublet of molecular weight 190,000 and 180,000 using, as a probe, anti-TCSA antibodies and antibodies against a synthetic peptide (TR) derived from the long tandem repeat domain of TCSA. Binding of the monoclonal and polyclonal antibodies to E. tenella was completely blocked by TR, but not by an irrelevant peptide (BR). Therefore, E. tenella expresses a developmentally regulated SA that is structurally related to the T. cruzi counterpart. Because of the high SA activity in merozoites, and by analogy with other SA-producing microbes that inhabit mucin-rich epithelia, we suggest that the Eimeria SA plays a role in desialylating intestinal mucins to reduce viscosity of the local environment and thereby facilitate parasite migration. The enzyme could also play a role in host cell-parasite interaction.

Mannitol metabolism in Eimeria tenella

Unsporulated oocysts of Eimeria tenella contain large quantities of carbohydrates, namely amylopectin, mannitol and glucose. Analysis of carbohydrate content of sporulating oocysts revealed that mannitol content increased markedly during early stages of sporogony (first 4-6h) but slowly diminished during the next 40h of sporulation. Accumulation of mannitol was accompanied by a rapid decrease in amylopectin and free glucose, suggesting that mannitol might be synthesized from glucose released from amylopectin. Mannitol was also detected in sporozoite and merozoite extracts. All four mannitol cycle enzymes were detected in oocysts. Sporozoites excysted in vitro had lower activities of all four enzymes. Mannitol-1-phosphatase and mannitol dehydrogenase activity was also detected in merozoites obtained from the second stage schizonts. Sporozoites incubated with 14C-glucose accumulated radioactively labelled precursor continuously for over 12h and some of the 14C-glucose was converted into 14C-mannitol. These results indicate that mannitol plays an important role in the metabolism and development of the intracellular stages of the parasite.

Resolution of three forms of superoxide dismutase by immobilised metal affinity chromatography

A new method for separation of three forms of superoxide dismutase (SOD) using immobilised metal affinity chromatography (IMAC) is reported. Fe-, Mn- and Cu/Zn-SODs were eluted sequentially from Cu(2+)-IMAC column with an increasing gradient of a counter ion (NH+4) run in combination with an increasing pH gradient (6.8-7.8). The combined gradient elution method resulted in separation of SODs with high resolution, the three proteins being eluted in electrophoretically homogeneous forms. Similar preparation could not be achieved by either increasing gradient of a counter ion or decreasing pH gradients used separately. The described methodology has been successfully applied for separation of three SODs from a protozoan parasite, indicating that this combined gradient elution system for IMAC offers new possibilities for the high-resolution separation of proteins exhibiting only minor differences in their amino acid composition and structure.

Estimation of relative fecundity in Eimeria tenella strains by a mixed infection method

The relative fecundity of populations of Eimeria tenella was estimated by means of mixed infection using electrophoretic variation of glucose-phosphate isomerases (GPIs) as a genetic marker. A decoquinate-resistant strain with GPI-9 isozyme (DR-NIAH), a decoquinate-sensitive one with GPI-1 (DS-Iwate), and three decoquinate-resistant lines (No. 3, 4, and 5) derived from cross-fertilization between DR-NIAH and DS-Iwate, were used. The GPI phenotypes of the No. 3 and No. 4 lines are GPI-9, and that of No. 5 is GPI-1. Mixed infection experiments were performed between DR-NIAH and DS-Iwate, No. 3 and No. 5, and No. 4 and No. 5. DR-NIAH was predominant over DS-Iwate in the mixed infection, whereas, in single infections, the total oocyst output of DR-NIAH was similar to or less than that of DS-Iwate. Among three lines, No. 4 was predominant over No. 5, and No. 5 was predominant over No. 3 in the mixed infection. Relative fecundity between No. 3 and No. 5 and their patterns of oocyst output in single infections were similar to those in the mixed infection. In contrast, No. 5 produced more oocysts than No. 4 in single infections, suggesting that the oocyst production in the mixed infection may be influenced by the mutual interference or competition between the populations of E. tenella in the chicken caeca.