'CURE' OF CRITHIDIA (STRIGOMONAS) ONCOPELTI OF ITS BACTERIAL ENDOSYMBIOTE

Biosynthesis of vitamins and cofactors in bacterium-harbouring trypanosomatids depends on the symbiotic association as revealed by genomic analyses

Some non-pathogenic trypanosomatids maintain a mutualistic relationship with a betaproteobacterium of the Alcaligenaceae family. Intensive nutritional exchanges have been reported between the two partners, indicating that these protozoa are excellent biological models to study metabolic co-evolution. We previously sequenced and herein investigate the entire genomes of five trypanosomatids which harbor a symbiotic bacterium (SHTs for Symbiont-Haboring Trypanosomatids) and the respective bacteria (TPEs for Trypanosomatid Proteobacterial Endosymbiont), as well as two trypanosomatids without symbionts (RTsfor Regular Trypanosomatids), for the presence of genes of the classical pathways for vitamin biosynthesis. Our data show that genes for the biosynthetic pathways of thiamine, biotin, and nicotinic acid are absent from all trypanosomatid genomes. This is in agreement with the absolute growth requirement for these vitamins in all protozoa of the family. Also absent from the genomes of RTs are the genes for the synthesis of pantothenic acid, folic acid, riboflavin, and vitamin B_6. This is also in agreement with the available data showing that RTs are auxotrophic for these essential vitamins. On the other hand, SHTs are autotrophic for such vitamins. Indeed, all the genes of the corresponding biosynthetic pathways were identified, most of them in the symbiont genomes, while a few genes, mostly of eukaryotic origin, were found in the host genomes. The only exceptions to the latter are: the gene coding for the enzyme ketopantoate reductase (EC:1.1.1.169) which is related instead to the Firmicutes bacteria; and two other genes, one involved in the salvage pathway of pantothenic acid and the other in the synthesis of ubiquinone, that are related to Gammaproteobacteria. Their presence in trypanosomatids may result from lateral gene transfer. Taken together, our results reinforce the idea that the low nutritional requirement of SHTs is associated with the presence of the symbiotic bacterium, which contains most genes for vitamin production.

Replication of kinetoplast DNA of Crithidia acanthocephali. I. Density shift experiments using deuterium oxide

The protozoan Crithidia acanthocephali contains, within a modified region of a mitochondrion, a mass of DNA known as kinetoplast DNA (kDNA). This DNA consists mainly of an association of approximately 27,000 covalently closed 0.8-mum circular molecules which are apparently held together in a definite ordered manner by topological interlocking. After culturing of C. acanthocephali cells for 25 generations in medium containing 75% deuterium oxide, both nuclear DNA (rhonative, nondeuterated=1.717 g/cm3) and kDNA (rhonative, nondeuterated=1.702 g/cm3) increased in buoyant density by 0.012 g/cm3. The replication of the two DNAs was studied by cesium chloride buoyant density analysis of DNAs from exponentially growing cells taken at 1.0, 1.4, 2.0, 3.0, and 4.0 cell doublings after transfer of cells from D2O-containing medium into medium containing only normal water. The results obtained from analysis of both native and denatured nuclear DNAs indicate that this DNA replicates semiconservatively. From an analysis of intact associations of kDNA, it appears that this DNA doubles once per generation and that the newly synthesized DNA does not segregate from parental DNA. Fractions of covalently closed single circular molecules and of open circular and unit length linear molecules were obtained from associations of kDNA by sonication, sucrose sedimentation, and cesium chloride-ethidium bromide equilibrium gradient centrifugation. Buoyant density profiles obtained from these fractions indicate that: (a) doubling of the kDNA results from the replication of each circular molecule rather than from repeated replication of a small fraction of the circular molecules; (b) replication of kDNA is semiconservative rather than conservative, but there is recombination between the circles at an undefined time during the cell cycle.

Symbiote-free hemoflagellates, Blastocrithidia culicis and Crithidia oncopelti: their liver factor requirement and serologic identity

Several aposymbiotic strains of Blastocrithidia culicis and Crithidia oncopelti were cultivated in Trager's chemically defined medium as well as in a blood broth, both supplemented with 0.25% (v/v) liver extract concentrate. For all such strains, the liver extract was found to serve as an essential growth factor in the defined medium and as growth promoting additive in the blood broth. The active molecules were found to be water-soluble, heat stable, dialyzable, and probably nonlipid fractions. Antisera were developed in rabbit against all the available aposymbiotic strains. An almost total cross-reactivity at very high titers was observed in reciprocal agglutination test using strains with and without the bacterial symbiotes. These results indicate that the loss of the symbiotes does not affect the antigenic identity of B. culicis and C. oncopelti.

Physicochemical properties of kinetoplast DNA from Crithidia acanthocephali. Crithidia luciliae, and Trypanosoma lewisi

The protozoa Crithidia and Trypanosoma contain within a mitochondrion a mass of DNA known as kinetoplast DNA (kDNA) which consists mainly of an association of thousands of small circular molecules of similar size held together by topological interlocking. Using kDNA from Crithidia acanthocephali, Crithidia luciliae, and Trypanosoma lewisi, physicochemical studies have been carried out with intact associations and with fractions of covalently closed single circular molecules, and of open single circular and unit length linear molecules obtained from kDNA associations by sonication, sucrose sedimentation, and cesium chloride-ethidium bromide equilibrium centrifugation. Buoyant density analyses failed to provide evidence for base composition heterogeneity among kDNA molecules within a species. The complementary nucleotide strands of kDNA molecules of all three species had distinct buoyant densities in both alkaline and neutral cesium chloride. For C. acanthocephali kDNA, these buoyant density differences were shown to be a reflection of differences in base composition between the complementary nucleotide strands. The molar ratios of adenine: thymine:guanine:cytosine, obtained from deoxyribonucleotide analyses were 16.8:41.0:28.1:14.1 for the heavy strand and 41.6:16.6:12.8:29.0 for the light strand. Covalently closed single circular molecules of C. acanthocephali (as well as intact kDNA associations of C. acanthocephali and T. lewisi) formed a single band in alkaline cesium chloride gradients, indicating their component nucleotide strands to be alkaline insensitive. Data from buoyant density, base composition, and thermal melting analyses suggested that minor bases are either rare or absent in Crithidia kDNA. The kinetics of renaturation of 32P labeled C. acanthocephali kDNA measured using hydroxyapatite chromatography were consistent with at least 70% of the circular molecules of this DNA having the same nucleotide sequence. Evidence for sequence homologies among the kDNAs of all three species was obtained from buoyant density analyses of DNA in annealed mixtures containing one component kDNA strand from each of two species.

Heme biosynthesis in bacterium-protozoon symbioses: enzymic defects in host hemoflagellates and complemental role of their intracellular symbiotes

Heme biosynthetic activity in the symbiotic association involving crithidial flagellates and intracellular bacteroids was studied by enzymic, nutritional, and isotope incorporation experiments. Component organisms and their complexes in this association were analyzed separately to determine the underlying cause of the hemin requirement of hemoflagellates and the role of symbiotes in sparing this requirement of two crithidial species. Nutritional study of symbiote-free flagellates showed that their growth requires at least 0.1 mug/ml of hemin, which can be substituted by protoporphyrin IX, but not by the porphyrin precursors, delta-amino-levulinic acid or porphobilinogen. These flagellates, in the presence of protoporphyrin IX, incorporated 59Fe into heme, indicating that they possess ferrochelatase (EC 4.99.1.1), the terminal enzyme in the heme biosynthetic pathway, which catalyzes the insertion of iron into protoporphyrin IX. In symbiote-containing flagellates serially cultured in a defined medium free of tetrapyrrole compounds, heme and porphyrins can be detected by a fluorophotometric method, indicative of heme biosynthesis. Study of [14C]glycine incorporation into heme showed that the rate is much higher in symbiote-containing flagellates than in those without symbiotes. Microassay of uroporphyrinogen I synthase [EC 4.3.1.8; porphobilinogen ammonia-lyase (polymerizing)] revealed that the specific activity is high in symbiote-containing flagellates and higher still in isolated symbiotes, but essentially negligible in symbiote-free organisms. It is concluded that the bacterial symbiotes augment a very limited heme biosynthetic capacity of host flagellates by supplying uroporphyrinogen I synthase and perhaps other enzymes preceding ferrochelatase in the heme biosynthetic chain.

Purification and properties of nucleic acids from an unusual cytoplasmic organelle in the flagellate protozoan Crithidia oncopelti

A simple, rapid method for preparing bipolar bodies from sonicated (rithidia oncopelti cells, with a yield of 2-5%, is described. Apart from 2-4% contamination with unbroken cells the fraction was considered pure with respect to contamination by other nucleic acid-containing organelles, as judged by light and electron microscopy. A light satellite DNA, f bouyant density 1.695 g/ml in neutral CsCl, and derived from the bipolar body, had a Tm of 81.6 degrees C in0.15 M NaCl/0.015 M sodium citrate (pH 7.0) and a kinetic complexity of 2.7 with 109. The bipolar body fraction also contained ribonucleoprotein particles with and s20,w of 67 S, in contrast to cytoplasmic ribosomes (87 S). Bipolar body ribosomes contained rRNA components which migraged coincidentally with Escherichia coli rRNA (molecular weights 1.07 with 10-6 and 0.56 with 10-6) on polyacrylamide gel electrophoresis. Cytoplasmic ribosomes contained rRNAs of molecular weights 1.30 with 10-6 and 0.83 with 10-6. Bipolar body rRNA accounted for up to 10% of the rRNA extracted from cells. The properties of these bipolar body nucleic acids provide good evidence for the bacterial nature of this subcellular component.

Nutritional significance of symbiotic bacteria in two species of hemoflagellates

Symbiote-free strains of Blastocrithidia culicis and Crithidia oncopelti, obtained by chloramphenicol treatment, were compared nutritionally with normal, symbiote-containing strains. The symbiotic bacteria spare the flagellates requirements for exogenous hemin and for other nutritional factors present in liver extract.

The form and structure of kinetoplast DNA of Crithidia

Cesium chloride centrifugation of each of the DNAs extracted from eight strains of Crithidia revealed a main band at rho = 1.717 g/cm(3) and a satellite band varying from rho = 1.701 to 1.705 g/cm(3) for the different strains By electron microscopy each DNA was shown to include circular molecules, 0.69-0.80 micro in mean contour length, and large, topologically two-dimensional masses of DNA in which the molecules appeared in the form of rosettes. DNA isolated from kinetoplast fractions of Crithidia acanthocephali was shown to consist of light satellite DNA and to be mainly in the form of large masses, 0.8 micro (mol wt = 1.54 x 10(6) daltons) circular molecules, and a few long, linear molecules. The results of experiments involving ultracentrifugation, heating, and quenching, sonication, and endodeoxyribonuclease digestion, combined with electron microscopy, are consistent with the following hypothesis. The large DNA masses are associations of 0.8 micro circles which are mainly covalently closed. The circles are held together in groups (the rosettes) of up to 46 by the topological interlocking of each circle with many other circles in the group. A group of circles is attached to an adjacent group by one or more circles, each interlocking with many circles of both groups. Each of the associations comprises, on the average, about 27,000 circles (total mol wt approximately 41 x 10(9) daltons). A model is proposed for the in situ arrangement of the associations which takes into consideration their form and structure, and appearance in thin sections