Episomally driven antisense mRNA abrogates the hyperinducible expression and function of a unique cell surface class I nuclease in the primitive trypanosomatid parasite, Crithidia luciliae

Here, we show that Crithidia luciliae, a primitive trypanosomatid, purine auxotroph, up-expressed its unique, bi-functional, surface membrane 3'-nucleotidase/nuclease (Cl 3'NT/NU) activity by approximately 1000-fold in response to purine starvation. A second surface membrane phospho-monoesterase, i.e. a tartrate-resistant acid phosphatase (Cl MAcP) was also found to be up-expressed in such purine-starved cells. Here, we used homologous episomal-expression of an antisense construct of the Cl3'NT/NU to dissect the functional expression of these two surface membrane enzymes. In antisense transfected cells, a large excess of the antisense transcript was produced and no trace of any endogenous Cl3'NT/NU sense message was detected. Further, the purine-starvation hyper-induced levels of 3'NT/NU enzyme activity were completely abrogated in these transfected cells versus controls. Moreover, such antisense transcription completely abolished the ability of these transfectants to grow in poly(A)-containing medium demonstrating the essential nature of the 3'NT/NU for the growth/survival of this parasite. In contrast, antisense transcription had no apparent deleterious effects on either endogenous or purine-starvation-induced levels of MAcP enzyme activity, its steady-state mRNA levels, or the constitutive expression of house-keeping genes (e.g. Cl alpha-tubulin) in these transfectants. Cumulatively, results of our antisense experiments demonstrated that the functional nuclease activity of the surface membrane Cl 3'NT/NU was, in fact, critical/essential for the growth and development of these primitive parasites.

An endosymbiont positively modulates ornithine decarboxylase in host trypanosomatids

Some trypanosomatids, such as Crithidia deanei, are endosymbiont-containing species. Aposymbiotic strains are obtained after antibiotic treatment, revealing interesting aspects of this symbiotic association. Ornithine decarboxylase (ODC) promotes polyamine biosynthesis and contributes to cell proliferation. Here, we show that ODC activity is higher in endosymbiont-bearing trypanosomatids than in aposymbiotic cells, but isolated endosymbionts did not display this enzyme activity. Intriguingly, expressed levels of ODC were similar in both strains, suggesting that ODC is positively modulated in endosymbiont-bearing cells. When the aposymbiotic strain was grown in conditioned medium, obtained after cultivation of the endosymbiont-bearing strain, cellular proliferation as well as ODC activity and localization were similar to that observed in the endosymbiont-containing trypanosomatids. Furthermore, dialyzed-heated medium and trypsin treatment reduced ODC activity of the aposymbiont strain. Taken together, these data indicate that the endosymbiont can enhance the protozoan ODC activity by providing factors of protein nature, which increase the host polyamine metabolism.

A metalloproteinase extracellularly released by Crithidia deanei

Actively motile cells from a cured strain of Crithidia deanei released proteins in phosphate buffer (pH 7.4). The molecular mass of the released polypeptides, which included some proteinases, ranged from 19 to 116 kDa. One of the major protein bands was purified to homogeneity by a combination of anion-exchange and gel filtration chromatographs. The apparent molecular mass of this protein was estimated to be 62 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The incorporation of gelatin into SDS-PAGE showed that the purified protein presented proteolytic activity in a position corresponding to a molecular mass of 60 kDa. The enzyme was optimally active at 37 degrees C and pH 6.0 and showed 25% of residual activity at 28 degrees C for 30 min. The proteinase was inhibited by 1,10-phenanthroline and EDTA, showing that it belonged to the metalloproteinase class. A polyclonal antibody to the leishmanial gp63 reacted strongly with the released C. deanei protease. After Triton X-114 extraction, an enzyme similar to the purified metalloproteinase was detected in aqueous and detergent-rich phases. The detection of an extracellular metalloproteinase produced by C. deanei and some other Crithidia species suggests a potential role of this released enzyme in substrate degradation that may be relevant to the survival of trypanosomatids in the host.

A novel extracellular calcium-dependent cysteine proteinase from Crithidia deanei

An extracellular cysteine proteinase from an aposymbiotic strain of Crithidia deanei was purified 39-fold by a combination of anion-exchange and gel filtration chromatographies. The native molecular mass of this proteinase was estimated to be 225 kDa by gel filtration chromatography and it migrates in SDS-PAGE as a single band of 80 kDa. The optimal enzymatic activity on gelatin was found to occur in the presence of calcium at a neutral pH and at 28 degrees C. The enzyme was completely blocked by E-64 and EGTA, and partially inhibited by iodoacetamide, leupeptin, and EDTA. Compounds such as PMSF, aprotinin, and pepstatin weakly inhibited the enzyme. The protein purified in the present work shares some features with those of the family of neutral calcium-dependent cysteine proteinases named calpains, previously detected in the family Trypanosomatidae as cell-associated enzymes in Leishmania donovani and Trypanosoma brucei. The cysteine proteinase from C. deanei is distinct from the well-characterized mammalian calpains, but some degree of similarity is displayed to invertebrate calpain-related enzymes.

Extracellular release of the surface metalloprotease, gp63, from Leishmania and insect trypanosomatids

Protease activity was found in spent culture medium collected from Leishmania donovani, L. mexicana, L. major, as well as the insect trypanosomatids, Crithidia luciliae and Leptomonas seymouri. Released protease activity increased linearly over time and was correlated to promastigote density. In SDS-PAGE, zymogram gels showed that the protease's molecular weight ranged from 43-100 kDa. Spent culture medium proteases were blocked by the metallo-protease inhibitors, 1,10-phenanthroline and Z-Tyr-Leu-NHOH, but not by bestatin, leupeptin, ABESF, pepstatin A, E-64 or aprotinin. Monoclonal and/or polyclonal antibodies to the leishmanial gp63 reacted with the released Crithidia, Leptomonas, L. major and L. donovani proteases. Cell surface biotinylation and immune precipitation using gp63-specific antibodies showed that >34% of the released protease originated from the surface. Antibodies against the Trypanosoma brucei variable surface glycoprotein cross-reactive determinant (CRD) did not recognize this activity, suggesting that the gp63 is not cleaved from the cell surface by a parasite phospholipase, but is released by an alternative mechanism.

Crithidia guilhermei: gelatin- and haemoglobin-degrading extracellular metalloproteinases

The extracellular metalloproteinases of the insect trypanosomatid Crithidia guilhermei were characterized through the incorporation of different protein substrates (gelatin, casein, haemoglobin, and bovine serum albumin) into SDS-PAGE. Two gelatinases (60 and 80 kDa) showed ability to degrade casein as well and a 67-kDa enzyme presented the broadest specificity since it was also able to degrade casein and haemoglobin. Besides the 67-kDa extracellular proteinases detected on haemoglobin-SDS-PAGE, a 43-kDa haemoglobinase was only observed with this substrate. All C. guilhermei proteinases were incapable of using bovine serum albumin. C. guilhermei was also grown in four different culture media and the best proteinase production was reached using yeast extract-peptone medium containing glucose as the major carbon source. The results point to the importance of the use of distinct culture media and proteinaceous substrates on the characterization of extracellular proteolytic activities in trypanosomatids, since alterations in growth conditions and methods of detection could lead to distinct proteolytic profiles.

Ectonucleotide diphosphohydrolase activity in Crithidia deanei

In this work we describe the ability of living Crithidia deanei to hydrolyze extracellular ATP. In intact cells at pH 7.2, a low level of ATP hydrolysis was observed in the absence of any divalent metal (0.41+/-0.13 nmol P(i) h(-1) 10(7) cells(-1)). The ATP hydrolysis was stimulated by MgCl(2) and the Mg(2+)-dependent ecto-ATPase activity was 4.05+/-0.17 nmol P(i) h(-1) 10(7) cells(-1). Mg(2+)-dependent ecto-ATPase activity increased linearly with cell density and with time for at least 60 min. The addition of MgCl(2) to extracellular medium increased the ecto-ATPase activity in a dose-dependent manner. At 5 mM ATP, half-maximal stimulation of ATP hydrolysis was obtained with 0.93+/-0.26 mM MgCl(2). This stimulatory activity was also observed when MgCl(2) was replaced by MnCl(2), but not CaCl(2) or SrCl(2). The apparent K(m) for Mg-ATP(2-) was 0.26+/-0.03 mM. ATP was the best substrate for this enzyme; other nucleotides, such as ITP, GTP, UTP and CTP, produced lower reaction rates. In the pH range from 6.6 to 8.4, in which the cells were viable, the acid phosphatase activity also present in this cell decreased, while the Mg(2+)-dependent ATPase activity did not change. This ecto-ATPase activity was insensitive to inhibitors of other ATPase and phosphatase activities, such as oligomycin, sodium azide, bafilomycin A(1), ouabain, vanadate, molybdate, sodium fluoride and tartrate. To confirm that this Mg(2+)-dependent ATPase was an ecto-ATPase, we used the impermeant inhibitor 4, 4'-diisothiocyanostylbene 2'-2'-disulfonic acid as well as suramin, an antagonist of P(2) purinoreceptors and inhibitor of some ecto-ATPases. These two reagents inhibited the Mg(2+)-dependent ATPase activity in a dose-dependent manner. The cell surface location of the ATP-hydrolyzing site was also confirmed by cytochemical analysis.

Ecto-phosphatase activity on the cell surface of Crithidia deanei

In the present work we have partially characterized an ecto-phosphatase activity in Crithidia deanei, using viable parasites. This enzyme hydrolyzed p-nitrophenylphosphate at a rate of 3.55 +/- 0.47 nmol Pi/h x 10(8) cells. The dependence on p-NPP concentration shows a normal Michaelis-Menten kinetics for this phosphatase activity and the value of the apparent Km for p-NPP was 5.35 +/- 0.89 mM. This phosphatase activity was inhibited by the product of the reaction, the inorganic phosphate. Experiments using classical inhibitors of acid phosphatases, such as ZnCl2 and sodium fluoride, as well as inhibitors of phosphotyrosine phosphatase, such as sodium orthovanadate and ammonium molybdate, showed a decrease in this phosphatase activity, with different patterns of inhibition.

Differential expression of proteolytic enzymes in endosymbiont-harboring Crithidia species

Crithidia oncopelti, Crithidia deanei and Crithidia desouzai are flagellates of the Trypanosomatidae family that present bacterium-like endosymbionts in their cytoplasm. Gelatin-SDS-PAGE analysis was used to characterize cell-associated and extracellular proteinases in these organisms. Our survey indicates that the proteolytic profiles of C. deanei and C. desouzai are identical; that C. oncopelti displays a distinct zymogram; and that species naturally lacking endosymbionts have a more complex extracellular proteolytic activity, which illustrates the heterogeneity of this genus. This is the first report on the presence of cysteine proteinases in the culture supernatant of monoxenic trypanosomatids, and by the use of wild and aposymbiotic strains from C. deanei we also demonstrated that the prokaryote endosymbiont somehow alters quantitatively the expression of extracellular proteinases in this trypanosomatid.

Crithidia guilhermei: purification and partial characterization of a 62-kDa extracellular metalloproteinase

An extracellular metalloproteinase from Crithidia guilhermei, a monoxenic trypanosomatid of insects, was purified 11-fold by ammonium sulfate precipitation, gel filtration on a Shinpack Diol-150 column, and anion-exchange chromatography in a MONO Q column, both using the HPLC system. The proteinase appeared as a single band with an apparent molecular mass of 62 kDa in SDS-PAGE, under reducing conditions, and was optimally active at 37 degrees C and pH 6.0. The enzyme showed 62% residual activity at 50 degrees C for 30 min. The proteinase was completely inhibited by 1, 10-phenanthroline, indicating that the enzyme belongs to the metalloproteinase class. This is the first report of the purification of an extracellular metalloproteinase from the Crithidia species. The possible role of this enzyme in the digestive tract of the insect host is discussed.

Molecular characterization of a hyperinducible, surface membrane-anchored, class I nuclease of a trypanosomatid parasite

The 3'-nucleotidase/nuclease (3'-NT/NU) is a surface enzyme unique to trypanosomatid parasites. These organisms lack the pathway for de novo purine biosynthesis and thus are entirely dependent upon their hosts to supply this nutrient for their survival, growth, and multiplication. The 3'-NT/NU is involved in the salvage of preformed purines via the hydrolysis of either 3'-nucleotides or nucleic acids. In Crithidia luciliae, this enzyme is highly inducible. For example, in these organisms purine starvation triggers an approximately 1000-fold up-expression of 3'-NT/NU activity. In the present study, we cloned and characterized a gene encoding this intriguing enzyme from C. luciliae (Cl). Sequence analysis showed that the Cl 3'-NT/NU deduced protein possessed five regions, which we defined here as being characteristic of members of the class I nuclease family. Further, we demonstrated that the Cl 3'-NT/NU-expressed protein possessed both 3'-nucleotidase and nuclease activities. Moreover, we showed that the dramatic up-expression of 3'-NT/NU activity in response to purine starvation of C. luciliae was concomitant with the approximately 100-fold elevation in steady-state mRNA specific for this gene. Finally, results of our nuclear run-on analyses demonstrated that such up-regulation in 3'-NT/NU enzyme activity was mediated at the posttranscriptional level.

Detection by HPLC of a trypanothione synthetase activity in vitro from Entamoeba histolytica

We have previously demonstrated the presence of glutathione-spermidine (Gsp) and trypanothione [T(SH)(2)] from Entamoeba histolytica trophozoites, on the basis of results obtained with acid extracts purified by Florisil and DEAE-cellulose, derivatized with the fluorescent reagent monobromobimane and separated by HPLC. Gsp was originally found in Escherichia coli and later in trypanosomatids such as Trypanosoma cruzi, T. brucei, T. congolense and the insect trypanosomatid Crithidia fasciculata, along with the novel compound T(SH)(2), N(1), N(8)-bis(glutathionyl)-spermidine. Here we demonstrate the presence of a T(SH)(2) synthetase activity in partly purified extracts from Entamoeba histolytica HK9, incubated at two pH values (6.5 and 7.5) with reduced glutathione (GSH), spermidine and ATP, in the presence of Mg(2+) at different time intervals. The thiol products were detected by HPLC in picomole amounts and compared with commercial Gsp and T(SH)(2) standards. We have used also an extract of Crithidia luciliae as a reference, to compare our results with C. fasciculata, in which the presence of this enzyme has previously been demonstrated and was later purified and separated into two synthetase activities from the same source: one for Gsp and the other for T(SH)(2). The presence of a T(SH)(2) synthetase activity in Entamoeba histolytica means that this protozoan has a similar metabolism to that of the trypanosomatids and opens the possibility of establishing a rational drug design against this human parasite.

The kinetoplast structure-specific endonuclease I is related to the 5' exo/endonuclease domain of bacterial DNA polymerase I and colocalizes with the kinetoplast topoisomerase II and DNA polymerase beta during replication

The mitochondrial DNA (kinetoplast DNA) of the trypanosomatid Crithidia fasciculata has an unusual structure composed of minicircles and maxicircles topologically interlocked into a single network and organized in a disc-shaped structure at the base of the flagellum. We previously purified a structure-specific endonuclease (SSE1), based on its RNase H activity, that is enriched in isolated kinetoplasts. The endonuclease gene has now been cloned, sequenced, and found to be closely related to the 5' exonuclease domain of bacterial DNA polymerase I proteins. Although the protein does not contain a typical mitochondrial leader sequence, the enzyme is shown to colocalize with a type II DNA topoisomerase and a DNA polymerase beta at antipodal sites flanking the kinetoplast disc. Cell synchronization studies with an epitope-tagged construct show that the localization of the endonuclease to the antipodal sites varies in a cell cycle-dependent manner similar to that of the DNA polymerase beta [Johnson, C. E. & Englund, P. T. (1998) J. Cell Biol. 143, 911-919]. Immunofluorescent localization of SSE1 to the antipodal sites is only observed during kinetoplast replication. Together, these results suggest a point of control for kinetoplast DNA replication through the regulation of the availability of DNA replication proteins and a possible role for the antipodal sites in removal of RNA primers and the repair of gaps in newly replicated minicircles.

Isolation of tubulin polyglutamylase from Crithidia; binding to microtubules and tubulin, and glutamylation of mammalian brain alpha- and beta-tubulins

Trypanosomatids have a striking cage-like arrangement of submembraneous microtubules. We previously showed that alpha- and beta- tubulins of these stable microtubules are extensively modified by polyglutamylation. Cytoskeletal microtubular preparations obtained by Triton extraction of Leishmania tarentolae and Crithidia fasciculata retain an enzymatic activity that incorporates radioactive glutamic acid in a Mg2+-ATP-dependent manner into alpha- and beta-tubulins. The tubulin polyglutamylase is extracted by 0.25 M salt. The Crithidia enzyme can be purified by ATP-affinity chromatography, glycerol-gradient centrifugation and ion-exchange chromatography. After extraction from the microtubular cytoskeleton the glutamylase forms a complex with alphabeta tubulin, but behaves after removal of tubulin as a globular protein with a molecular mass of 38x10(3). In highly enriched fractions a corresponding band is the major polypeptide visible in SDS-PAGE. The enzyme from Crithidia recognises mammalian brain tubulin, where it incorporates glutamic acid preferentially into the more acidic variants of both alpha- and beta-tubulins. Synthetic peptides with an oligoglutamyl side chain, corresponding to the carboxy-terminal end of brain alpha- and beta-tubulins, are accepted by the enzyme, albeit at low efficiency. The polyglutamylase elongates the side chain by up to 3 and 5 residues, respectively. Other properties of the tubulin polyglutamylase are also discussed.

Ultrastructural and biochemical analysis of the relationship of Crithidia deanei with its endosymbiont

Some protozoa of the Trypanosomatidae family harbor in their cytoplasm bacterial endosymbionts that provide essential nutrients to and induce morphological alterations in the protozoa. In the present study, a close association between endosymbionts and glycosomes, a peroxisome-like organelle where most of the enzymes of the glycolytic pathway are compartmentalized, was identified by conventional transmission electron microscopy in Crithidia deanei. Such an association was further supported by the cytochemical localization of catalase in the glycosome and also confirmed by 3-D reconstruction of the protozoan. The enzymes cytochrome oxidase and succinate dehydrogenase were detected by ultrastructural cytochemistry. A positive reaction was observed in the protozoan mitochondrion but not in the endosymbiont envelope. Enzymatic assays for succinate cytochrome c reductase reinforced these results, as a low enzymatic activity was detected in an endosymbiont-enriched fraction, while high activity was observed in a purified protozoan mitochondrion fraction. We also demonstrated that a purified symbiont fraction was able to hydrolyze ATP. This activity was Mg+2 dependent, since it was highly stimulated by the presence of physiological concentrations of this ion. Taken together, these observations suggest that no electron transporting system is active in the symbionts of Crithidia deanei and that they might obtain energetic molecules derivated from the protozoan glycosomes.

Crithidia luciliae: regulation of purine nucleoside transport by extracellular purine concentrations

During the growth cycle of the protozoan parasite Crithidia luciliae, there was a dramatic concomitant increase in the rate of adenosine and guanosine transport and 3' nucleotidase (3'NTase) activity after 72-94 hr. The simultaneous increased activities of the nucleoside transporters and 3'NTase could be suppressed by addition to the medium of a purine supplement such as adenosine (100 microM). C. luciliae grown in purine-replete medium (> or = 75 microM adenosine) exhibited low rates of adenosine and guanosine transport whilst parasites transferred to a defined serum-free medium containing < or = 7.5 microM adenosine demonstrated elevated levels of both adenosine and guanosine transport up to 25- to 40-fold. The increased activity of the nucleoside transporters was inhibited by cycloheximide (10 microM). Under conditions of purine depletion 3'AMP and 3'GMP inhibited the adenosine and guanosine transporters, respectively. However, in the presence of a purine supplement (100 microM), neither 3'AMP nor 3'GMP was an effective inhibitor of nucleoside transport. Our results link the increased activity of the nucleoside transporters to the increased activity of the 3'NTase, indicating the activation of a purine salvage system not previously reported in other organisms.

Crithidia luciliae: effect of purine starvation on S-adenosyl-L-methionine uptake and protein methylation

The utilization of S-adenosyl-L-[methyl-3H]methionine ([3H-methyl]AdoMet) by Crithidia luciliae was assessed under nutrient-replete and purine-starvation conditions. Uptake experiments with intact cells demonstrated that the radiolabel from this molecule was accumulated by purine-starved organisms at a rate approximately 10-fold greater than that observed in those cultivated in nutrient-replete medium. Purine-starved cells also incorporated the radiolabel into trichloroacetic acid insoluble material at an approximately 10-fold faster rate than nutrient-replete cells. No differences, however, were observed in the intracellular levels of AdoMet and its metabolites between organisms cultivated under the two conditions. Results of comparative labeling studies with [3H-methyl]AdoMet, S-adenosyl-L-[carboxyl-14C]methionine, L-[methyl-3H]methionine and L-[35S]methionine in the presence and absence of cycloheximide demonstrated that the incorporation of label from [3H-methyl]AdoMet was due to transmethylation and was independent of protein synthesis. Further, approximately 15 methylated protein bands were identified by SDS-PAGE analysis. Lysates from both purine-starved and nutrient-replete organisms demonstrated similar levels of activity of three protein methyltransferases (PMI, II, III). The differences observed in [3H-methyl]AdoMet utilization between purine-starved and nutrient-replete C. luciliae may reflect the enhanced purine transport capacity which results from purine starvation.

Properties of uracil phosphoribosyltransferase from Giardia intestinalis

Incorporation of pyrimidine ribonucleotides in Giardia intestinalis occurs via uracil phosphoribosyltransferase (UPRTase). The enzyme was purified over 1000-fold to apparent homogeneity from parasite extracts, using Fast Protein Liquid Chromatography, namely Mono Q anion exchange, Mono P chromatofocusing and Superose 12 chromatography. The specific activity of the purified enzyme was 3100 nmol min-1 mg protein-1. The enzyme was found to be a dimer of mol. wt. 76,000. Kinetic analysis, including initial velocity and product inhibition studies, indicated that it obeyed a rapid-random equilibrium mechanism. GTP and dGTP caused a dramatic increase in the activity of the enzyme, though there was no effect on the Michaelis constants. All other nucleotides tested were without effect or were inhibitory. The effect of GTP is similar to that observed for UPRTase from E. coli but not from other eukaryotes.

Amidrazone analogues of D-ribofuranose as transition-state inhibitors of nucleoside hydrolase

The transition state of inosine during hydrolysis by nucleoside hydrolase has been characterized by kinetic isotope effects, bond-energy/bond-order vibrational analysis, and molecular electrostatic potential surface calculations [Horenstein, B. A., Parkin, D. W., Estupinan, B., & Schramm, V. L. (1991) Biochemistry 30, 10788-10795; Horenstein, B. A., & Schramm, V. L. (1993) Biochemistry 32, 7089-7097]. The heterocyclic base is protonated and the anomeric carbon of the ribofuranosyl ring is flattened to form a transition-state with extensive oxocarbenium ion character. With their delocalized charge and flattened structures, amidrazone analogues of D-ribofuranose provide both geometric and electronic mimics of the ribosyl group at the transition-state of nucleoside hydrolase. A family of riboamidrazones was synthesized with H, phenyl, and p-nitrophenyl N-substituents. The analogues were competitive inhibitors with respect to inosine and gave Ki values of 10(-5), 2 x 10(-7), and 1 x 10(-8) M, respectively. (p-Nitrophenyl)riboamidrazone exhibited slow-onset, tight-binding inhibition, with an overall dissociation constant of 2 x 10(-9) M. The binding is reversible with an off-rate of 3 x 10(-3) s-1. Tight binding can be attributed to the close spatial match between the molecular geometry of (p-nitrophenyl)riboamidrazone and the transition-state stabilized by nucleoside hydrolase. The favorable binding interactions of the (p-nitrophenyl)riboamidrazone include oxocarbenium ion mimicry, isosteric ribosyl hydroxyls, and hydrophobic and H-bonding interactions at the nitrophenyl group.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate specificity of the purine-2'-deoxyribonucleosidase of Crithidia luciliae

The purine-2'-deoxyribonucleosidase of Crithidia luciliae catalyses an efficient deoxyribosyl transfer between a variety of purine bases, benzimidazole and 5,6-dimethylbenzimidazole. Since the deoxyriboside of a deoxyribosyl acceptor is necessarily also a substrate, the trans-N-deoxyribosylase activity of the enzyme allows a study of its specificity to be extended to a large number of purines and purine analogues. Amongst 27 different deoxyribosyl acceptors, only hypoxanthine gave rise to isomeric products. The introduction of methyl groups at appropriate positions in either purine or benzimidazole lowered the Michaelis constant, KB, for deoxyribosyl acceptors: by about 10-fold for 6-methylpurine (KB 351 +/- 87 microM) compared with purine (KB 3.91 +/- 0.8 mM) and by about 10(3)-fold for 5,6-dimethylbenzimidazole (KB 7.0 +/- 0.79 microM) compared with benzimidazole (Km,app. 7.8 +/- 2.4 mM). The maximal rates of deoxyribosyl transfer to different acceptors, on the other hand, varied by only 4.5-fold, and can be ascribed to decreases in the rate of release of the newly formed purine deoxyriboside from the enzyme.

X-ray structure of trypanothione reductase from Crithidia fasciculata at 2.4-A resolution

Trypanosomes and related protozoan parasites lack glutathione reductase and possess instead a closely related enzyme that serves as the reductant of a bis(glutathione)-spermidine conjugate, trypanothione. The human and parasite enzymes have mutually exclusive substrate specificities, providing a route for the design of therapeutic agents by specific inhibition of the parasite enzyme. We report here the three-dimensional structure of trypanothione reductase from Crithidia fasciculata and show that it closely resembles the structure of human glutathione reductase. In particular, the core structure surrounding the catalytic machinery is almost identical in the two enzymes. However, significant differences are found at the substrate binding sites. A cluster of basic residues in glutathione reductase is replaced by neutral, hydrophobic, or acidic residues in trypanothione reductase, consistent with the nature of the spermidine linkage and the change in overall charge of the substrate from -2 to +1, respectively. The binding site is more open in trypanothione reductase due to rotations of about 4 degrees in the domains that form the site, with relative shifts of as much as 2-3 A in residue positions. These results provide a detailed view of the residues that can interact with potential inhibitors and complement previous modeling and mutagenesis studies on the two enzymes.

Herpetomonas roitmani (Fiorini et al., 1989) n. comb.: a trypanosomatid with a bacterium-like endosymbiont in the cytoplasm

The trypanosomatid previously described as Crithidia roitmani is characterized here at the ultrastructural and biochemical levels. The data indicates that the parasite belongs to the Herpetomonas genus, and we therefore suggest the flagellate to be denominated as Herpetomonas roitmani n. comb. Cladistic analysis of isoenzyme data generated by eight different enzymes showed that the parasite presented a distinct banding pattern and could be grouped with some Herpetomonas spp., but not with Crithidia spp., used as reference strains. Accordingly, when the parasites were grown for longer periods in Roitman's defined medium, expontaneous differentiation from promastigotes to opisthomastigotes (typical of the Herpetomonas genus) occurred. Transmission electron microscopy revealed the presence of bacterium-like endosymbionts in the cytoplasm of all evolutive forms of the parasite. All morphological alterations characteristic of endosymbiont-bearing trypanosomatids could be observed.

Purification and characterization of the 3'-nucleotidase/nuclease from promastigotes of Leishmania donovani

The surface membrane-associated 3'-nucleotidase/nuclease (3'-N'ase) of Leishmania donovani has been purified from detergent extracted promastigotes by anion and cation exchange, lectin affinity and gel filtration chromatography. SDS-PAGE analysis of the purified enzyme preparation revealed a 43-kDa polypeptide as well as faster migrating bands. These bands co-migrated, following both one- and two-dimensional electrophoretic analyses, with enzyme activity as determined by an in situ 3'-nucleotidase gel activity assay. It is suggested that the lower molecular weight species arise during purification as a result of proteolytic cleavage of the intact 43-kDa enzyme. The 3'-N'ase exhibited a pI of 5.4, as revealed by 2-dimensional gel electrophoresis. The glycoprotein nature of the 3'-N'ase was suggested by its binding to concanavalin A and by its electrophoretic shift following incubation with N-glycanaseR. In nucleotidase and nuclease assays, the 3'-N'ase was most active with 3'-AMP and poly(A), respectively. Both nucleotidase and nuclease activities exhibited broad pH optima with peaks at 8.5 and 7.5, respectively. At pH 8.5 nucleotidase activity was inhibited by EDTA, Zn2+ and thiols, but was insensitive to tartrate, molybdate and fluoride ions, commonly used inhibitors of phosphatases. The properties of the leishmanial 3'-N'ase was similar to the 3'-N'ase purified from purine-starved Crithidia luciliae, a related trypanosomatid protozoan, and to group of nucleases from fungi and germinating plant seedlings.

Inhibition of polyamine biosynthesis in Crithidia fasciculata by D,L-alpha-difluoromethylornithine and D,L-alpha-difluoromethylarginine

Using Crithidia fasciculata as a model organism for Trypanosoma cruzi, we have examined the effects of D,L-alpha-difluoromethylornithine (DFMO) and D,L-alpha-difluoromethylarginine (DFMA) on growth and polyamine synthesis. In a defined, polyamine-free medium growth was markedly inhibited by DFMO (94% at 50 mM; IC50 = 37 mM) and to a lesser extent by DFMA (65% at 50 mM). Addition of putrescine, but not agmatine, reverses inhibition of growth, suggesting that the site of inhibition is ornithine decarboxylase (ODC). Consistent with this conclusion, DFMO or DFMA results in a complete loss of putrescine and significant reductions in intracellular spermidine, glutathionylspermidine and N1,N8-bis(glutathionyl)spermidine (trypanothione). In addition, significant concentrations of DFMO (0.8 mM) were present in DFMA-treated cells. However, in contrast to other organisms, conversion of DFMA to DFMO is probably not catalysed by arginase. Substantial ornithine decarboxylase activity (63.1 pmol min-1 mg-1; ODC) was observed in control cells, sufficient to account for polyamine synthesis during growth. In addition, a trace arginine decarboxylase (ADC) activity (1.19 pmol min-1 mg-1) was found. Evidence is presented showing that the apparent ADC activity is actually due to the concerted action of arginase (1.5 nmol min-1 mg-1) and ODC. Thus DFMA appears to inhibit growth of C. fasciculata via conversion to DFMO and subsequent inhibition of ODC.

The purine-2-deoxyribonucleosidase from Crithidia luciliae. Purification and trans-N-deoxyribosylase activity

Crude extracts of Crithidia luciliae catalysed a deoxyribosyl transfer from purine deoxynucleosides to free purine bases. Fractionation of a 0-80% (NH4)2SO4 fraction from C. luciliae on DEAE-cellulose resulted in the separation of three nucleosidase activities. Two of these were ribonucleosidases, one specific for inosine, uridine and xanthosine and the other for inosine and guanosine, whereas the third activity was specific for purine deoxyribonucleosides. This pattern is similar to that found in Leishmania donovani. Significant deoxyribosyltransferase activity was, however, associated with the purine-2'-deoxyribonucleosidase from C. luciliae. The purine-2'-deoxyribonucleosidase was purified to homogeneity by a six-step procedure involving (NH4)2SO4 fractionation and chromatography on DEAE-cellulose, hydroxyapatite, Sephadex G-75, and a chromatofocusing resin. The purified enzyme migrated as a single band of 17 kDa on SDS/polyacrylamide gel electrophoresis. The enzyme catalysed the hydrolysis of deoxyinosine, deoxyguanosine and deoxyadenosine with Km values of 80 +/- 10.5 microM, 20.7 +/- 3.2 microM and 17.3 +/- 5.3 microM, respectively, and V values for these substrates in the ratio 1:0.5:0.39. The pH optimum for deoxyribosyl transfer from deoxyinosine to guanine was at pH 7.7, while deoxyinosine hydrolysis in the presence of guanine was optimal in the range pH 6-7. During the synthesis of deoxyinosine from hypoxanthine and deoxyadenosine two products were formed. One of these coeluted with deoxyinosine on HPLC, while the second was tentatively identified as the positional isomer, 7-(beta-D-2'-deoxyribofuranosyl)hypoxanthine.

Characterization of the RNA polymerases of Crithidia fasciculata

We have characterized the crithidial RNA polymerases (RNAPs). The RNAPs from this organism were resolved by chromatography and could be classified into RNAP I, II and III because of their differential sensitivity to class-specific RNA polymerase inhibitors, such as alpha-amanitin (ama) and tagetitoxin. The three RNAP classes were subsequently characterized in nuclear run-ons using class-specific DNA templates and the inhibitory effect of ama on RNA chain elongation. These experiments showed that Crithidia fasciculata contains the normal set of eukaryotic RNAPs. However, RNAP II was found to be relatively resistant to ama, which seems to be a general feature of kinetoplastid parasites. Tagetitoxin, a potent inhibitor of purified crithidial RNAP III, does not inhibit RNAP III in nuclear run-on experiments, suggesting that the inhibitory effect of tagetitoxin depends on the enzyme conformation, i.e. 'naked' enzyme versus that in a transcription complex. Finally, the role of RNAP III in the transcription of the mini-exon genes, which provide the 5' end of each mRNA, is discussed.

Class I and class II ribonuclease H activities in Crithidia fasciculata (Protozoa)

The protozoan Crithidia fasciculata contains two different ribonuclease H activities. These enzymes display similar physical and biochemical characteristics to their homologues in higher eukaryotes, for instance calf thymus class I and class II ribonuclease H. Class I ribonuclease H of lower and higher eukaryotes can be activated by Mg2(+)- and Mn2(+)-ions. However, the presence of Mn2(+)-ions is inhibitory for the Mg2(+)-dependent class II ribonuclease H activity of Crithidia fasciculata and calf thymus. The protozoan class I-homologue enzyme appears to be serologically related to the class I ribonuclease H of calf thymus.

Initiating a crystallographic study of trypanothione reductase

We have obtained well-ordered single crystals of the flavoenzyme trypanothione reductase from Crithidia fasciculata. The crystals are tetragonal rods with unit cell dimensions a = 128.6 A, c = 92.5 A. The diffraction pattern corresponds to a primitive lattice. Laue class 4/m. Diffraction to better than 2.4 A has been recorded at the Daresbury Synchrotron. The accurate elucidation of the three-dimensional structure of this enzyme is required to support the rational design of compounds active against a variety of tropical diseases caused by trypanosomal parasites.

Separation and quantitation of the polyamine biosynthesis inhibitor D,L-alpha-difluoromethylarginine and other guanidine-containing compounds by high-performance liquid chromatography

The arginine decarboxylase inhibitor difluoromethylarginine (DFMA) is an important tool in the study of polyamine metabolism, particularly with respect to the human pathogen Trypanosoma cruzi. This paper demonstrates a unique method for the detection and quantitation of intracellular DFMA using the fluorogenic agent 9,10-phenanthrenequinone. After separation of cell extracts by HPLC, DFMA can be accurately and reproducibly quantified with a lower sensitivity limit of 0.1 nmol by this simple fluorometric method. This assay can also be used to detect other guanidine-containing compounds such as arginine, agmatine, creatinine, and hirudonine, but not substituted guanidines such as aminoguanidine and creatine, or the structurally related amidines such as benzamidine and pentamidine.

Preliminary crystallographic analysis of trypanothione reductase from Crithidia fasciculata

Trypanothione reductase, a flavoprotein disulfide reductase specific to trypanosomatid parasites, has been crystallized by vapor diffusion of a protein solution (10 mg/ml) against 22% polyethylene glycol (average Mr 8000) containing 100 mM-ammonium sulfate. Crystals of a size suitable for structure determination by X-ray diffraction have been obtained by seeding protein solutions with smaller crystals. The space-group is P21 (a = 60.9 A, b = 161.8 A, c = 58.4 A, beta = 99.1 degrees). The molecular mass and volume of the unit cell suggest that there is a dimer of the enzyme in the asymmetric unit, and this is confirmed by self-rotation functions calculated using data to 4.5 A resolution. The crystals diffract to beyond 3 A resolution. Crystals of another P21 form (a = 91.3 A, b = 114.4 A, c = 92.0 A, beta = 141.3 degrees) are observed to grow under similar conditions.

Substrate specificity and inhibitor sensitivity of a trypanosomatid alkaline peptidase

An alkaline peptidase of Trypanosoma cruzi and Crithidia fasciculata, previously shown to cleave on the carboxyl side of arginine and lysine residues, was examined for its ability to cleave various fluorogenic substrates and for its sensitive to peptidase inhibitors. The enzyme of both T. cruzi and C. fasciculata has a preference for cleavage of substrates with basic amino acids at the P2 as well as the P1 position of the peptide chain. Arginine and lysine are equally acceptable at P2, whereas the enzyme prefers arginine to lysine at P1. An influence of the P3 amino acid residue on substrate cleavability was also apparent. The peptidase was highly susceptible to inactivation by peptidylfluoromethanes, peptidyldiazomethanes and peptidylsulphonium salts that contained arginine or lysine at P1. Additionally, diisopropylfluorophosphate inhibited the enzyme, whereas trans-epoxysuccinylleucylamido(4-guanidino)butane and iodoacetic acid were relatively weak inhibitors. Various reversible inhibitors of the enzyme were also examined. Inhibition by members of the primary aliphatic amine series, methylamine to n-heptylamine, showed a peak of inhibition at n-butylamine, which most closely resembles the lysine side chain. Agmatine, which resembles the arginine side chain, also strongly inhibited the peptidase. The kinetics of inhibition by these basic compounds were of the competitive type. Pentamidine and hirudonin, which resemble two arginine side chains joined together, were more effective inhibitors of the trypanosomatid peptidase than bases resembling only one arginine or lysine side chain.

Crithidia luciliae: starvation for purines and/or phosphate leads to the enhanced surface expression of a protein responsible for 3'-nucleotidase/nuclease activity

It has been shown previously that starvation of the trypanosomatid protozoan Crithidia luciliae for purines and/or inorganic phosphate results in increased levels of a surface membrane-associated 3'-nucleotidase/nuclease (3'-N'ase) activity which hydrolyzes both 3'-ribonucleotides and nucleic acids, thereby permitting the organisms to transport these essential nutrients across their cell membranes. A polypeptide with the requisite catalytic properties has been identified by an in situ gel activity assay following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In current studies, differential synthesis of the protein responsible for the 3'-N'ase activity was not demonstrable by comparisons of SDS-PAGE patterns of nutrient-replete or purine-starved parasites metabolically labeled with either [35S]methionine, [3H]leucine, or [3H]tyrosine. However, surface labeling of nutrient-replete and purine-starved cells revealed the enhanced expression of an 125I surface-labeled 43-kDa protein which comigrated with the 3'-N'ase activity in one- and two-dimensional electrophoretic systems. The amount of this surface-labeled peptide correlated with the level of 3'-N'ase activity as measured by test tube assay. Refeeding adenosine to purine-starved cells led to the loss of both the enzyme activity and the surface iodinatable 43-kDa band as a result of renewed cell division. Starvation of these organisms for phosphate also led to the enhanced expression of the 43-kDa radioiodinatable band. The results indicated that the 3'-N'ase protein, itself, is differentially expressed at the cell surface under conditions which lead to increased enzyme activity.

An inducible 3'-nucleotidase/nuclease from the trypanosomatid Crithidia luciliae. Purification and characterization

Several species of protozoan parasites of the family Trypanosomatidae have a surface membrane-associated enzyme which is capable of hydrolyzing extracellular 3'-nucleotides and nucleic acids, thereby aiding in the acquisition of nutritionally required purines and Pi from their hosts. In Crithidia luciliae, this 3'-nucleotidase/nuclease previously has been shown to be highly regulated as purine and/or Pi starvation of this trypanosomatid leads to as much as a 1000-fold increase in enzyme activity. We have purified the enzyme to apparent homogeneity from detergent extracts of purine-starved C. luciliae by heparin-agarose chromatography followed by Mono Q and Mono S fast protein liquid chromatography. The enzyme had an apparent molecular weight of 43,000 and a pI of approximately 5.8. The enzyme displayed broad pH optima, with peaks at 8.0, for both nucleotidase and nuclease activities. The pH optima shifted to lower values when the activity was assayed in the presence of sulfhydryl reagents. The enzyme was most active with 3'-AMP and poly(A) in nucleotidase and nuclease assays, respectively. As a nuclease the enzyme hydrolyzed RNA at a faster rate than single-stranded DNA with no detectable hydrolysis of double-stranded DNA. The loss of enzyme activity which occurred upon storage at acid pH was prevented by the inclusion of Zn2+ in storage buffers. The physicochemical and kinetic properties of this trypanosomatid enzyme suggest that it is similar to the class I nucleases found in fungi and in germinating seedlings of higher plants.

Decreased nuclear matrix DNA topoisomerase II in human leukemia cells resistant to VM-26 and m-AMSA

CEM leukemia cells selected for resistance to VM-26 (CEM/VM-1) are cross-resistant to various other DNA topoisomerase II inhibitors but not to Vinca alkaloids. Since DNA topoisomerase II is a major protein of the nuclear matrix, we asked if alterations in nuclear matrix topoisomerase II might be important in this form of multidrug resistance. Pretreatment of drug-sensitive CEM cells for 2 h with either 5 microM VM-26 or 3 microM m-AMSA reduced the specific activity of newly replicated DNA on the nuclear matrix by 75 and 50%, respectively, relative to that of the bulk DNA. However, neither VM-26 nor m-AMSA affected the relative specific activity of nascent DNA isolated from the nuclear matrices of drug-resistant CEM/VM-1 cells. The decatenating and unknotting activities of DNA topoisomerase II were 6- and 7-fold lower, respectively, in the nuclear matrix preparations from the CEM/VM-1 cells compared to parental CEM cells. Western blot analysis revealed that the amount of immunoreactive topoisomerase II in the nuclear matrices of the CEM/VM-1 cells was decreased 3.2-fold relative to that in CEM cells, but there was no significant difference in the amount of enzyme present in the nonmatrix (1.5 M salt soluble) fractions of nuclei from these cell lines. Increasing the NaCl concentration used in the matrix isolation procedure from 0.2 to 1.8 M resulted in a progressive decrease in the specific activity of topoisomerase II in matrices of CEM/VM-1 but not CEM cells, which suggested that the association of the enzyme with the matrix is altered in the resistant cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Biosynthesis of the trypanosomatid metabolite trypanothione: purification and characterization of trypanothione synthetase from Crithidia fasciculata

Trypanothione synthetase from Crithidia fasciculata has been purified ca. 14,500-fold to homogeneity in an overall yield of 40%. The pure enzyme catalyzed the synthesis of N1- and N8-glutathionylspermidine and N1,N8-bis(glutathionyl)spermidine (trypanothione) from ATP/magnesium, glutathione (GSH), and spermidine, N1- and N8-glutathionylspermidines being intermediates of trypanothione synthesis. The enzyme showed a sharp pH optimum of 7.5-7.75 for the synthesis of both mono- and diglutathionylspermidine conjugates. It was highly specific for its physiological substrates ATP/Mg2+, GSH, spermidine, and N1- and N8-glutathionylspermidine with Km values of 400 microM, 914 microM, 1.07 mM, 20 microM, and 7 microM, respectively. Trypanothione synthetase was active in the monomeric form with Mr = 87,000 and absorption maxima lambda max = 225 and 280 nm (A280/A260 = 1.85). Trypanothione synthetase is a new member of the ATP-dependent class of ligases which form amide linkage with concomitant production of ADP and orthophosphate.

Purification and characterization of uracil phosphoribosyltransferase from Crithidia luciliae

1. Uracil phosphoribosyltransferase (UPRTase) was purified 370-fold from the protozoan parasite, Crithidia luciliae. 2. The enzyme was a dimer of mol. wt 80 000 and was highly specific for uracil. 3. GTP, which is an activator of UPRTase from E. coli had a slight inhibitory effect on the parasite enzyme. 4. The C. luciliae UPRTase demonstrated a broad specificity for activating divalent metal ions.

Characterization of a novel endonuclease from Crithidia fasciculata

A new endonuclease activity has been identified in whole cell lysates of the trypanosomatid Crithidia fasciculata. This activity, termed endonuclease A (Endo A), introduces single-strand breaks at highly preferred sites in double stranded DNA substrates Physical analysis of this enzyme indicates that it has a sedimentation coefficient S20,W of 4.9 and a Stokes radius of 59A and thus, a native molecular weight of 125,000 and a frictional coefficient of 1.8. A monomeric structure is suggested for the enzyme based on the recovery of Endo A activity associated with a polypeptide with a molecular weight of 116,000-120,000, following electrophoresis on sodium dodecyl sulfate polyacrylamide gels. Endo A shows an absolute requirement for Mg2+ or Mn2+ and exhibits activity over a broad pH and temperature range, with optimal conditions for activity at pH 8.0 and 30 degrees C.

Unusual C-terminal domain of the largest subunit of RNA polymerase II of Crithidia fasciculata

The C-terminal domain of the largest subunit of RNA polymerase II in higher eukaryotes is present in the protozoan parasite Trypanosoma brucei in a strongly modified form. To determine whether this is a general feature of the Kinetoplastida and to determine the role of this domain in RNA polymerase II transcription, we have analysed the C-terminal domain of the distantly related species Crithidia fasciculata. No positional identity of amino acid residues between the C-termini of C. fasciculata and T. brucei can be found. Moreover, both domains lack the heptapeptide repeat structure present in higher eukaryotes. The two domains are, however, very similar in amino acid composition, being rich in acidic residues as well as serine and tryosine. The latter observation is compatible with the concept that in vivo phosphorylation of the C-terminus activates RNA polymerase II.

Isolation of the peptide inhibitor of H+-ATP synthase from Crithidia fasciculata and Trypanosoma cruzi

An inhibitor of Crithidia fasciculata and Trypanosoma cruzi H+ -ATP synthase (ATPase) was isolated from these organims mitochondrial particles, either by (a) ammonium sulfate-cholate extraction followed by heat treatment and ethanol precipitation, or (b) gel-filtration on Sephadex G-50, followed by a similar purification procedure. Inactivation by trypsin supported the inhibitor peptide structure. Removal of the peptide inhibitor increased about three-fold the specific activity of the protozoan ATPases. The isolated peptides and a highly purified bovine heart ATPase inhibitor inhibited C. fasciculata ATPase as a function of the peptide concentration.

Localization of a type II DNA topoisomerase to two sites at the periphery of the kinetoplast DNA of Crithidia fasciculata

A type II DNA topoisomerase (topollmt), purified to near homogeneity from the trypanosomatid C. fasciculata has been shown to be localized to the single mitochondrion of these kinetoplastid protozoa. Immunoblots show at least a 10-fold higher level of topollmt (per milligram of protein) in preparations of partially purified mitochondria as compared with those from whole cells. Analyses of type I and type II topoisomerase activities in both mitochondrial and whole cell extracts show a 4- to 5-fold higher specific activity of topollmt in mitochondrial extracts while a nuclear type I topoisomerase has a 4- to 5-fold lower specific activity in the same extract. Immunolocalizations using anti-topollmt antibodies show the enzyme to be present in close association with the mitochondrial DNA networks (kinetoplast DNA or kDNA). This association appears at two distinct locations on opposite sides of the kDNA network.

Bent DNA structures associated with several origins of replication are recognized by a unique enzyme from trypanosomatids

Sequence-directed bending of the DNA double helix is a conformational variation found in both prokaryotic and eukaryotic organisms. The utilization of bent DNA structures from various sources as specific signals recognized by an enzyme is demonstrated here using a unique endonuclease purified from trypanosomatid cells. Crithidia fasciculata nicking enzyme was previously shown to recognize specifically the bent structure found in kinetoplast DNA minicircles. The binding constant measured for this specific interaction is of two orders of magnitude higher than that measured for the binding of the enzyme to a non-curved sequence. As determined by binding competition and mobility shift electrophoresis analyses, this enzyme recognizes the sequence-directed bends associated with the origins of replication of bacteriophage lambda and simian virus 40 (SV40), as well as that located within the autonomously replicating sequence (ARS1) region of the yeast S. cerevisiae.

Crithidia luciliae: factors affecting the expression of 3'-nucleotidase/nuclease activity

Crithidia luciliae, a trypanosomatid protozoan readily grown in axenic cultures, was shown to possess low levels of a surface membrane-bound ectoenzyme capable of hydrolyzing both 3'-ribonucleotides and nucleic acids. The specific activities of this 3'-nucleotidase/nuclease, with both mononucleotide and nucleic acid substrates, were greatly enhanced when the protozoa were deprived of purines, an essential nutrient. The catalytic activities were exhibited by a polypeptide which migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an Mr of 47,000. Starvation of these cells for inorganic phosphate (Pi), in media with or without purines, also led to an increase in the specific activity of the ectoenzyme compared to that of Pi- and purine-replete cells. In contrast, the level of enzyme activity was not increased when the protozoa were starved, under purine-replete conditions, for either arginine or hemin, two other essential nutrients. Cells starved simultaneously for either of the latter two nutrients and for purines also did not show increased levels of the 3'-nucleotidase/nuclease. The activation of the enzyme was also prevented by sodium arsenite, cycloheximide, actinomycin D, and tunicamycin indicating that the activation presumably required metabolic energy as well as new transcription, translation, and protein modification. The results demonstrate that the control of 3'-nucleotidase/nuclease expression is a regulated, adaptive response to growth-limiting levels of essential nutrients.

The topogenic signal of the glycosomal (microbody) phosphoglycerate kinase of Crithidia fasciculata resides in a carboxy-terminal extension

To determine how microbody proteins enter microbodies, we have previously compared the genes for the cytosolic and glycosomal (microbody) phosphoglycerate kinases (PGKs) of Trypanosoma brucei and found the microbody enzyme to differ from other PGKs and the cytosolic form in two respects: a high net positive charge and a C-terminal extension of 20 amino acids (Osinga et al., 1985). Here we present the comparison of the genes for the cytosolic and glycosomal PGKs of Crithidia fasciculata, another kinetoplastid organism. The amino acid sequences of the two Crithidia isoenzymes are virtually identical, except for a C-terminal extension of 38 amino acids. We conclude that this extension must direct the glycosomal PGK to the glycosome. The extensions of the Crithidia and Trypanosoma enzymes are both rich in small hydrophobic and hydroxyl amino acids.

A unique endonuclease from Crithidia fasciculata which recognizes a bend in the DNA helix. Specificity of the cleavage reaction

The introduction of a single nick in DNA circles by Crithidia fasciculata nicking enzyme (Shlomai, J., and Linial, M. (1986) J. Biol. Chem. 261, 16219-16225) requires the presence of a bent structure in the DNA helix. However, the sequence directing the local bending of the DNA helix is not per se a preferred site for nicking by the enzyme. No extensive sequence specificity is involved in defining the cleavage site for C. fasciculata nicking enzyme in the duplex circular DNA substrate. However, the abundance of A and T residues is significantly high at both the 3' and the 5' termini generated at the nicked site. Nicking of the sequence-directed bent fragment from C. fasciculata kinetoplast DNA minicircles correlates with the periodicity determined by the unique nucleotide distribution in the bent sequence, reflected in its thermodynamic parameters. Occurrence of nicking is best correlated with the predicted minima of the melting temperature and delta G profiles, as well as with A and T dinucleotide sequences at the nicked site, in both the supercoiled and the relaxed sequence-directed bent DNA substrates. The potential role of the bend-dependent nicking reaction in the replication of kinetoplast DNA minicircles is discussed.

Sequence-directed bent DNA helix is the specific binding site for Crithidia fasciculata nicking enzyme

The sequence-directed bent structure of kinetoplast DNA minicircles specifies a unique binding site for Crithidia fasciculata nicking enzyme. Binding of the purified enzyme to the bent structure results in the formation of a tight enzyme-DNA complex that is highly specific to curved DNA. Recognition of the binding site is not determined by the nucleotide sequence at the site of binding per se but through the specific local variation in the DNA helix geometry. Both dynamic curved structures, which are generated by supercoiling, and static ones, which are sequenced-directed, could support and efficient enzyme-DNA complex formation. Binding interactions are dependent upon the degree of the helix curvature and decrease with the straightening of the binding site. DNase I protection experiments identify distinct domains of enzyme binding within the bent structure and suggest the induction of structural changes within these regions as a result of protein-DNA interactions.

The sequence-directed bent structure in kinetoplast DNA is recognized by an enzyme from Crithidia fasciculata

Crithidia fasciculata nicking enzyme (Shlomai, J., and Linial, M. (1986) J. Biol. Chem. 261, 16219-16225) interrupts a single phosphodiester bond in duplex DNA circles from various sources, only in their supercoiled form, but not following their relaxation by DNA topoisomerases. However, this requirement for DNA substrate supercoiling was not observed using the natural kinetoplast DNA as a substrate. Relaxed kinetoplast DNA minicircles, either free or topologically linked, were efficiently nicked by the enzyme. Furthermore, bacterial plasmids, containing a unit length kinetoplast DNA minicircle insert, were used as substrates for nicking in their relaxed form. This capacity to activate a relaxed DNA topoisomer as a substrate for nicking is an intrinsic property of the sequence-directed bend, naturally present in kinetoplast DNA. The 211-base pair fragment of the bent region from C. fasciculata kinetoplast DNA could support the nicking of a relaxed DNA substrate in a reaction dependent upon the DNA helix curvature.

Synthesis of dolichol derivatives in trypanosomatids. Characterization of enzymatic patterns

We have previously described that in certain parasitic protozoa, namely the trypanosomatids, the dolichol-P-P-linked oligosaccharides synthesized in vivo and transferred to protein are devoid of glucose residues and contain 6, 7, or 9 mannose units depending on the species. We have now conducted a cell-free characterization of the enzymatic patterns responsible for these phenotypes. Microsomes from Trypanosoma cruzi, Crithidia fasciculata, Leishmania enriettii, and Blastocrithidia culicis were found to synthesize dolichol-P-[14C]Man but not dolichol-P-[14C]Glc when incubated with rat liver dolichol-P and GDP-[14C]Man or UDP-[14C]Glc, thus providing for an explanation to the absence of glucosylated dolichol-P-P derivatives. Formation of dolichol-P-P-oligosaccharides was assayed in incubation mixtures containing rat liver dolichol-P, GDP-[14C]Man, microsomes, and unlabeled Man5-8GlcNAc2-P-P-dolichol from bovine liver. Membranes from species synthesizing dolichol-P-P-linked Man6GlcNAc2 or Man7GlcNAc2 in vivo were found to synthesize the same compounds but not the higher homologues in the cell-free assay. Species forming Man9GlcNAc2-P-P-dolichol in vivo were found to synthesize lipid-linked Man7GlcNAc2, Man8GlcNAc2, and Man9GlcNAc2 in vitro. It is concluded that there are at least three and probably four different dolichol-P-Man-dependent enzymatic activities involved in the synthesis of dolichol-P-P-linked Man9GlcNAc2 and that microorganisms not forming that compound are devoid of all mannosyltransferases responsible for the addition of the missing residues and not only of the enzyme involved in the synthesis of the homologue higher than the oligosaccharide occurring in vivo by a single mannose unit.

Purification and characterization of a tyrosine aminotransferase from Crithidia fasciculata

Tyrosine aminotransferase (TAT, EC 2.6.1.5) from the kinetoplastid, Crithidia fasciculata, was purified over 2000 fold to electrophoretic homogeneity. The native form of the enzyme had a molecular weight of approximately 100,000, whereas under denaturing conditions it produced two polypeptides of approximately 50,000 and 48,000, respectively. Absence of a reaction with the periodic acid-Schiff stain suggested that the crithidial enzyme was not a glycoprotein. It was relatively stable and remained active over a wide range of pH and temperature. It exhibited a broad substrate specificity and was able to utilize L-tyrosine, L-tryptophan, and L-phenylalanine as amino donors. Antiserum produced against partially purified crithidial tyrosine aminotransferase failed to inhibit the enzymatic activity. The same antiserum cross-reacted with a soluble extract from Trypanosoma brucei gambiense, but not with that from normal mouse liver, confirming evolutionary conservatism between the two protozoa.