Alpha and beta subunits of pyruvate dehydrogenase E1 from the microsporidian Nosema locustae: mitochondrion-derived carbon metabolism in microsporidia

Invertebrate host responses to microsporidia infections

Microsporidia are a group of fungi-like intracellular and unicellular parasites, which infect nearly all animals. As "master parasites", over 1400 microsporidian species have been described to date. Microsporidia infections in economical invertebrates (e.g., silkworm, shrimp) cause huge financial losses, while other microsporidia infections in daphnia, nematode, locust, honeybee and mosquito play important roles in the regulation of their population size. Research investigating invertebrate host responses following microsporidia infections has yielded numerous interesting results, especially pertaining to the innate immune response to these pathogens. In this review, we comparatively summarize the invertebrate host responses to various microsporidia infections. We discuss numerous critical events in host responses including ubiquitin-mediated resistance, production of reactive oxygen species, melanization and innate immune pathways, and the increased basic metabolism and the accumulation of juvenile hormone in infected hosts. Recent studies progressing our understanding of microsporidia infection are also highlighted. Collectively, these advances shed more light on general rules of invertebrate host immune responses and pathogenesis mechanisms of microsporidia, and concurrently offer valuable clues for further research on the crosstalk between hosts and intracellular pathogens.

An integrated metabolic consequence of Hepatospora eriocheir infection in the Chinese mitten crab Eriocheir sinensis

Despite the economic and evolutionary importance of aquatic host-infecting microsporidian species, at present, limited information has been provided about the microsporidia-host interactions. This study focused on Hepatospora eriocheir, an emerging microsporidian pathogen for the Chinese mitten crab Eriocheir sinensis. Hypertrophy of hepatopancreas cells was a common feature of H. eriocheir infection. More importantly, mitochondria of the hepatopancreas were drawn around the H. eriocheir, most likely to aid the uptake of ATP directly from the host. To better understand the crab anti-microsporidian response, de novo transcriptome sequencing of the hepatopancreas tissue was furtherly proceeded. A total of 47.84 M and 57.21 M clean reads were generated from the hepatopancreas of H. eriocheir infected and control groups respectively. Based on homology searches, functional annotation with 6 databases (Nr, Swiss-Prot, KEGG, KOGs, Pfam and GO) for 88,168 unigenes was performed. 2619 genes were identified as differently up-regulated and 2541 genes as differently down-regulated. Prominent functional categories enriched with differentially expressed genes (DEGs) were "ATP binding", "mitochondrion and extracellular region", "oxygen transporter activity", "oxidoreductase activity", "alanine, aspartate and glutamate metabolism", "carbohydrate metabolic process", "starch and sucrose metabolism" and "fatty acid biosynthesis". These results confirmed a parasite external energy supply and an integrated metabolic stress. In addition, simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs) were also identified from the gene library. Taken together, these findings allow us to better understand the underlying mechanisms regulating interactions between H. eriocheir and the crab E. sinensis.

New evidence on the relationship between Microsporidia and Fungi: a genome-wide analysis by DarkHorse software

Microsporidia are a group of obligate intracellular eukaryotic parasites that infect a wide variety of species, including humans. Phylogenetic analysis indicates a relationship between the Microsporidia and the Fungi. However, most results are based on the analysis of relatively few genes. DarkHorse analysis involves the transformation of BLAST results into a lineage probability index (LPI) value and allows for the comparison of genes for an entire genome with those of other genomes. Thus, we can see which genes from the microsporidia score most closely based on the LPI with other eukaryotic organisms. In this analysis, we calculated the LPI for each gene from the genomes of 7 Microsporidia, Antonospora locustae, Enterocytozoon bieneusi, Encephalitozoon cuniculi, Encephalitozoon intestinalis, Nosema bombycis, Nosema ceranae, and Nematocida parisii, to analyze the genetic relationships between Microsporidia and other species. It was found that many (91%) genes were most closely correlated with genes from other microsporidial genomes and had the highest mean LPI (0.985), indicating a monophyletic origin of the Microsporidia. In a subsequent analysis, we excluded the other Microsporidia from the analysis to look for relationships before the divergence of Microsporidia, and found that 43% of the microsporidial genes scored highest with fungal genes, and a higher mean LPI was found with Fungi than with other kingdoms, suggesting that Microsporidia is closely related to Fungi at the genomic level. Microsporidial genes were functionally clustered based on the KOG (Eukaryotic COG) database, and the possible lineages for each gene family were discussed in concert with the DarkHorse results.

Phylogenetic characterization of a microsporidium (Nosema sp.) isolated from the mulberry pest, Hemerophila atrilineata

Microsporidia are a group of obligate intracellular unicellular eukaryotes that can parasitize a wide variety of other eukaryotes ranging from protists to invertebrates and vertebrates. In this study, we examined the microsporidium Nosema sp. isolated from the mulberry pest, Hemerophila atrilineata Butler, 1881, named herein "Nosema sp. HA". The fresh spores were long oval in shape, 3.8 +/- 0.4 microm in length and 1.9 +/- 0.3 microm in width. Analysis of tissue infection of silkworm, Bombyx mori Linnaeus, 1758, indicated that the midgut, Malpighian tubules, muscle, fat body, silk glands, hemocytes, nerve tissue and gonads of silkworm were infected with Nosema sp. HA. The complete rRNA gene sequence of this microsporidium contained 4 305 base pairs (GenBank Accession JN882299), including the large subunit rRNA (2492 bp), the internal transcribed spacer (187 bp), the small subunit rRNA (1232 bp), the intergenic spacer (279 bp) and the 5S region (115 bp). The organization of the rRNA gene is 5'-LSU-ITS-SSU-IGS-5S-3'. Phylogenetic analysis, comparison of sequence identities and the arrangement in the rRNA gene subunits suggested that this isolate is separate from other Nosema species.

Recent developments in the taxonomic affiliation and phylogenetic positioning of fungi: impact in applied microbiology and environmental biotechnology

The goal of modern taxonomy is to understand the relationships of living organisms in terms of evolutionary descent. Thereby, the relationships between living organisms are understood in terms of nested clades--every time a speciation event takes place, two new clades are produced. Life comprises three domains of living organisms, these are the Bacteria, the Archaea and the Eukaryota. Within the eukaryotic domain, the fungi form a monophyletic group of the eukaryotic crown group and are thus high up in the evolutionary hierarchy of life. Fungus-like organisms possess certain morphological features of fungi, such as the hyphal organization of the Oomycota or the spores and reproductive structures inside a fructification of plasmodiophorids (Plasmodiophoromycota) and slime moulds (Mycetozoa). The first group are algae which secondarily lost their plastids during evolution and contain cellulose in their cell walls. Both osmotrophic phyla, the Oomycota and the Plasmidiophoromycota belong to the Chromista and Rhizaria, respectively, whereas the last group, the cellular and plasmodial slime moulds (Mycetozoa) are phagotrophic amoeboid protists belonging to the Amoebozoa. These fungus-like organisms are not considered further in this review. The Fungi sensu stricto comprise a heterogenous, often inconspicuous group of microorganisms which (1) are primarily heterotrophic with an (2) osmotrophic style of nutrition containing (3) chitin and its derivatives in the cell wall. This review discusses species concepts and current strategies in fungal taxonomy, phylogenetic affiliations of miscellaneous fungus-like groups like the microsporidia, perspectives of fungal nomenclature, and their impact on natural product research.

Chromalveolates and the evolution of plastids by secondary endosymbiosis

The establishment of a new plastid organelle by secondary endosymbiosis represents a series of events of massive complexity, and yet we know it has taken place multiple times because both green and red algae have been taken up by other eukaryotic lineages. Exactly how many times these events have succeeded, however, has been a matter of debate that significantly impacts how we view plastid evolution, protein targeting, and eukaryotic relationships. On the green side it is now largely accepted that two independent events led to plastids of euglenids and chlorarachniophytes. How many times red algae have been taken up is less clear, because there are many more lineages with red alga-derived plastids (cryptomonads, haptophytes, heterokonts, dinoflagellates and apicomplexa) and the relationships between these lineages are less clear. Ten years ago, Cavalier-Smith proposed that these plastids were all derived from a single endosymbiosis, an idea that was dubbed the chromalveolate hypothesis. No one observation has yet supported the chromalveolate hypothesis as a whole, but molecular data from plastid-encoded and plastid-targeted proteins have provided strong support for several components of the overall hypothesis, and evidence for cryptic plastids and new photosynthetic lineages (e.g. Chromera) have transformed our view of plastid distribution within the group. Collectively, these data are most easily reconciled with a single origin of the chromalveolate plastids, although the phylogeny of chromalveolate host lineages (and potentially Rhizaria) remain to be reconciled with this plastid data.

Particularities of mitochondrial structure in parasitic protists (Apicomplexa and Kinetoplastida)

Without mitochondria, eukaryotic cells would depend entirely on anaerobic glycolysis for ATP generation. This also holds true for protists, both free-living and parasitic. Parasitic protists include agents of human and animal diseases that have a huge impact on world populations. In the phylum Apicomplexa, several species of Plasmodium cause malaria, whereas Toxoplasma gondii is a cosmopolite parasite found on all continents. Flagellates of the order Kinetoplastida include the genera Leishmania and Trypanosoma causative agents of human leishmaniasis and (depending on the species) African trypanosomiasis and Chagas disease. Although clearly distinct in many aspects, the members of these two groups bear a single and usually well developed mitochondrion. The single mitochondrion of Apicomplexa has a dense matrix and many cristae with a circular profile. The organelle is even more peculiar in the order Kinetoplastida, exhibiting a condensed network of DNA at a specific position, always close to the flagellar basal body. This arrangement is known as Kinetoplast and the name of the order derived from it. Kinetoplastids also bear glycosomes, peroxisomes that concentrate enzymes of the glycolytic cycle. Mitochondrial volume and activity is maximum when glycosomal is low and vice versa. In both Apicomplexa and trypanosomatids, mitochondria show particularities that are absent in other eukaryotic organisms. These peculiar features make them an attractive target for therapeutic drugs for the diseases they cause.

Evidence of a reduced and modified mitochondrial protein import apparatus in microsporidian mitosomes

Microsporidia are a group of highly adapted obligate intracellular parasites that are now recognized as close relatives of fungi. Their adaptation to parasitism has resulted in broad and severe reduction at (i) a genomic level by extensive gene loss, gene compaction, and gene shortening; (ii) a biochemical level with the loss of much basic metabolism; and (iii) a cellular level, resulting in lost or cryptic organelles. Consistent with this trend, the mitochondrion is severely reduced, lacking ATP synthesis and other typical functions and apparently containing only a fraction of the proteins of canonical mitochondria. We have investigated the mitochondrial protein import apparatus of this reduced organelle in the microsporidian Encephalitozoon cuniculi and find evidence of reduced and modified machinery. Notably, a putative outer membrane receptor, Tom70, is reduced in length but maintains a conserved structure chiefly consisting of tetratricopeptide repeats. When expressed in Saccharomyces cerevisiae, EcTom70 inserts with the correct topology into the outer membrane of mitochondria but is unable to complement the growth defects of Tom70-deficient yeast. We have scanned genomic data using hidden Markov models for other homologues of import machinery proteins and find evidence of severe reduction of this system.

Morphological and molecular studies of a microsporidium (Nosema sp.) isolated from the thee spot grass yellow butterfly, Eurema blanda arsakia (Lepidoptera: Pieridae)

A microsporidium possessing molecular and morphological characteristics of the genus Nosema was isolated from larvae of the thee-spot grass yellow butterfly, Eurema blanda arsakia. The complete rRNA gene sequences of the E. blanda isolate contained 4,428 base pairs (GenBank Accession No. EU338534). The organization of the rRNA genes is LSU rRNA-ITS-SSU rRNA-IGS-5S, which corresponds with that of Nosema species closely related to Nosema bombycis. Phylogenetic analysis based on rRNA gene sequences show that this isolate is closely related to Nosema bombycis, Nosema plutellae, Nosema spodopterae, and Nosema antheraeae. The ultrastructure of all developmental stages of this microsporidium confirmed its placement in the genus Nosema. The isolate was successfully propagated in cell lines IPLB-LD652Y (Lymantria dispar) and NTU-LY (Lymantria xylina) and, in the in vitro system, it was frequently found to develop in the nuclei of the host cells, a circumstance that seldom occurs in other Nosema species. An extra-cellular vegetative stage of this microsporidium was also observed in the culture medium after 14 days of infection. The ECMDFs might be released from disrupted host cells.

Protein targeting in parasites with cryptic mitochondria

Many highly specialised parasites have adapted to their environments by simplifying different aspects of their morphology or biochemistry. One interesting case is the mitochondrion, which has been subject to strong reductive evolution in parallel in several different parasitic groups. In extreme cases, mitochondria have degenerated so much in physical size and functional complexity that they were not immediately recognised as mitochondria, and are now referred to as 'cryptic'. Cryptic mitochondrion-derived organelles can be classified as either hydrogenosomes or mitosomes. In nearly all cases they lack a genome and all organellar proteins are nucleus-encoded and expressed in the cytosol. The same is true for the majority of proteins in canonical mitochondria, where the proteins are directed to the organelle by specific targeting sequences (transit peptides) that are recognised by translocases in the mitochondrial membrane. In this review, we compare targeting sequences of different parasitic systems with highly reduced mitochondria and give an overview of how the import machinery has been modified in hydrogenosomes and mitosomes.

Microsporidian mitosomes retain elements of the general mitochondrial targeting system

Microsporidia are intracellular parasites that infect a variety of animals, including humans. As highly specialized parasites, they are characterized by a number of unusual adaptations, many of which are manifested as extreme reduction at the molecular, biochemical, and cellular levels. One interesting aspect of reduction is the mitochondrion. Microsporidia were long considered to be amitochondriate, but recently a tiny mitochondrion-derived organelle called the mitosome was detected. The molecular function of this organelle remains poorly understood. The mitosome has no genome, so it must import all its proteins from the cytosol. In other fungi, the mitochondrial protein import machinery consists of a network series of heterooligomeric translocases and peptidases, but in microsporidia, only a few subunits of some of these complexes have been identified to date. Here, we look at targeting sequences of the microsporidian mitosomal import system and show that mitosomes do in some cases still use N-terminal and internal targeting sequences that are recognizable by import systems of mitochondria in yeast. Furthermore, we have examined the function of the inner membrane peptidase processing enzyme and demonstrate that mitosomal substrates of this enzyme are processed to mature proteins in one species with a simplified processing complex, Antonospora locustae. However, in Encephalitozoon cuniculi, the processing complex is lost altogether, and the preprotein substrate functions with the targeting leader still attached. This report provides direct evidence for presequencing processing in mitosomes and also shows how a complex molecular system has continued to degenerate throughout the evolution of microsporidia.

Multiple secondary origins of the anaerobic lifestyle in eukaryotes

Classical ideas for early eukaryotic evolution often posited a period of anaerobic evolution producing a nucleated phagocytic cell to engulf the mitochondrial endosymbiont, whose presence allowed the host to colonize emerging aerobic environments. This idea was given credence by the existence of contemporary anaerobic eukaryotes that were thought to primitively lack mitochondria, thus providing examples of the type of host cell needed. However, the groups key to this hypothesis have now been shown to contain previously overlooked mitochondrial homologues called hydrogenosomes or mitosomes; organelles that share common ancestry with mitochondria but which do not carry out aerobic respiration. Mapping these data on the unfolding eukaryotic tree reveals that secondary adaptation to anaerobic habitats is a reoccurring theme among eukaryotes. The apparent ubiquity of mitochondrial homologues bears testament to the importance of the mitochondrial endosymbiosis, perhaps as a founding event, in eukaryotic evolution. Comparative study of different mitochondrial homologues is needed to determine their fundamental importance for contemporary eukaryotic cells.

Assessing the microsporidia-fungi relationship: Combined phylogenetic analysis of eight genes

Microsporidia are unicellular eukaryotes that are obligate parasites of a variety of animals. For many years, microsporidia were thought to be an early offshoot of the eukaryotic evolutionary tree, and early phylogenetic work supported this hypothesis. More recent analyses have consistently placed microsporidia far from the base of the eukaryotic tree and indicate a possible fungal relationship, but the nature of the microsporidian-fungal relationship has yet to be determined. The concatenated dataset employed in this analysis consists of eight genes and contains sequence data from representatives of four fungal phyla. A consistent branching pattern was recovered among four different phylogenetic methods. These trees place microsporidia as a sister to a combined ascomycete+basidiomycete clade. AU tests determined that this branching pattern is the most likely, but failed to reject two alternatives.

Complete mitochondrial genomes of the three brown algae (Heterokonta: Phaeophyceae) Dictyota dichotoma, Fucus vesiculosus and Desmarestia viridis

We report the complete mitochondrial sequences of three brown algae (Dictyota dichotoma, Fucus vesiculosus and Desmarestia viridis) belonging to three phaeophycean lineages. They have circular mapping organization and contain almost the same set of mitochondrial genes, despite their size differences (31,617, 36,392 and 39,049 bp, respectively). These include the genes for three rRNAs (23S, 16S and 5S), 25-26 tRNAs, 35 known mitochondrial proteins and 3-4 ORFs. This gene set complements two previously studied brown algal mtDNAs, Pylaiella littoralis and Laminaria digitata. Exceptions to the very similar overall organization include the displacement of orfs, tRNA genes and four protein-coding genes found at different locations in the D. dichotoma mitochondrial genome. We present a phylogenetic analysis based on ten concatenated genes (7,479 nucleotides) and 29 taxa. Stramenopiles were always monophyletic with heterotrophic species at the base. Results support both multiple primary and multiple secondary acquisitions of plastids.

E1 enzyme of the pyruvate dehydrogenase complex in Corynebacterium glutamicum: molecular analysis of the gene and phylogenetic aspects

The E1p enzyme is an essential part of the pyruvate dehydrogenase complex (PDHC) and catalyzes the oxidative decarboxylation of pyruvate with concomitant acetylation of the E2p enzyme within the complex. We analyzed the Corynebacterium glutamicum aceE gene, encoding the E1p enzyme, and constructed and characterized an E1p-deficient mutant. Sequence analysis of the C. glutamicum aceE gene and adjacent regions revealed that aceE is not flanked by genes encoding other enzymes of the PDHC. Transcriptional analysis revealed that aceE from C. glutamicum is monocistronic and that its transcription is initiated 121 nucleotides upstream of the translational start site. Inactivation of the chromosomal aceE gene led to the inability to grow on glucose and to the absence of PDHC and E1p activities, indicating that only a single E1p enzyme is present in C. glutamicum and that the PDHC is essential for the growth of this organism on carbohydrate substrates. Surprisingly, the E1p enzyme of C. glutamicum showed up to 51% identity to homodimeric E1p proteins from gram-negative bacteria but no similarity to E1 alpha- or beta-subunits of heterotetrameric E1p enzymes which are generally assumed to be typical for gram-positives. To investigate the distribution of E1p enzymes in bacteria, we compiled and analyzed the phylogeny of 46 homodimeric E1p proteins and of 58 alpha-subunits of heterotetrameric E1p proteins deposited in public databases. The results revealed that the distribution of homodimeric and heterotetrameric E1p subunits in bacteria is not in accordance with the rRNA-based phylogeny of bacteria and is more heterogeneous than previously assumed.

Comparative genomics of microsporidia

Microsporidia have been known for some time to possess among the smallest genomes of any eukaryote. There is now a completely sequenced microsporidian genome, as well as several other large-scale sequencing efforts, so the nature of these genomes is becoming apparent. This paper reviews some of the characteristics of microsporidian genomes in general, and some of the recent discoveries made through comparative genomic analyses. In general, microsporidian genomes are both reduced and compacted. Reduction takes place through gene loss, which is understandable in obligate intracellular parasites that rely on their host for many metabolites. Compaction is a more complex process, and is as yet not fully understood. It is clear from genomes surveyed thus far that the remaining genes are tightly packed and that there is little non-coding sequence, resulting in some extraordinary arrangements, including overlapping genes. Compaction also seems to affect certain aspects of genome evolution, like the frequency of rearrangements. The force behind this compaction is not known, and is especially interesting in light of the fact that surveys of genomes that are significantly different in size yield similar complements of protein-coding genes. There are some interesting exceptions, including catalase, photolyase and some mitochondrial proteins, but the rarity of these raises an interesting question as to what accounts for the significant differences seen in the genome sizes among microsporidia.

Phylogenetic analysis of the complete genome sequence of Encephalitozoon cuniculi supports the fungal origin of microsporidia and reveals a high frequency of fast-evolving genes

Microsporidia are unicellular eukaryotes living as obligate intracellular parasites. Lacking mitochondria, they were initially considered as having diverged before the endosymbiosis at the origin of mitochondria. That microsporidia were primitively amitochondriate was first questioned by the discovery of microsporidial sequences homologous to genes encoding mitochondrial proteins and then refuted by the identification of remnants of mitochondria in their cytoplasm. Various molecular phylogenies also cast doubt on the early divergence of microsporidia, these organisms forming a monophyletic group with or within the fungi. The 2001 proteins putatively encoded by the complete genome of Encephalitozoon cuniculi provided powerful data to test this hypothesis. Phylogenetic analysis of 99 proteins selected as adequate phylogenetic markers indicated that the E. cuniculi sequences having the lowest evolutionary rates preferentially clustered with fungal sequences or, more rarely, with both animal and fungal sequences. Because sequences with low evolutionary rates are less sensitive to the long-branch attraction artifact, we concluded that microsporidia are evolutionarily related to fungi. This analysis also allowed comparing the accuracy of several phylogenetic algorithms for a fast-evolving lineage with real rather than simulated sequences.

Class II photolyase in a microsporidian intracellular parasite

Photoreactivation is the repair of DNA damage induced by ultraviolet light radiation using the energy contained in visible-light photons. The process is carried out by a single enzyme, photolyase, which is part of a large and ancient photolyase/cryptochrome gene family. We have characterised a photolyase gene from the microsporidian parasite, Antonospora locustae (formerly Nosema locustae) and show that it encodes a functional photoreactivating enzyme and is expressed in the infectious spore stage of the parasite's life cycle. Sequence and phylogenetic analyses show that it belongs to the class II subfamily of cyclobutane pyrimidine dimer repair enzymes. No photolyase is present in the complete genome sequence of the distantly related microsporidian, Encephalitozoon cuniculi, and this class of photolyase has never yet been described in fungi, the closest relatives of Microsporidia, raising questions about the evolutionary origin of this enzyme. This is the second environmental stress enzyme to be found in A.locustae but absent in E.cuniculi, and in the other case (catalase), the gene is derived by lateral transfer from a bacterium. It appears that A.locustae spores deal with environmental stress differently from E.cuniculi, these results lead to the prediction that they are more robust to environmental damage.

Transfer of Nosema locustae (Microsporidia) to Antonospora locustae n. comb. based on molecular and ultrastructural data

Nosema locustae is a microsporidian parasite of grasshopper pests that is used as a biological control agent, and is one of the emerging model systems for microsporidia. Due largely to its diplokaryotic nuclei, N. locustae has been classified in the genus Nosema, a large genus with members that infect a wide variety of insects. However, some molecular studies have cast doubt on the validity of certain Nosema species, and on the taxonomic position of N. locustae. To clarify the affinities of this important insect parasite we sequenced part of the rRNA operon of N. locustae and conducted a phylogenetic analysis using the complete small subunit rRNA gene. Nosema locustae is only distantly related to the nominotypic N. bombycis, and is instead closely related to Antonospora scoticae, a recently described parasite of bees. We examined the ultrastructure of mature N. locustae spores, and found the spore wall to differ from true Nosema species in having a multi-layered exospore resembling that of Antonospora (one of the distinguishing features of that genus). Based on both molecular and morphological evidence, therefore, we propose transferring N. locustae to the genus Antonospora, as Antonospora locustae n. comb.

Genome compaction and stability in microsporidian intracellular parasites

Microsporidian genomes are extraordinary among eukaryotes for their extreme reduction: although they are similar in form to other eukaryotic genomes, they are typically smaller than many prokaryotic genomes. At the same time, their rates of sequence evolution are among the highest for eukaryotic organisms. To explore the effects of compaction on nuclear genome evolution, we sequenced 685,000 bp of the Antonospora locustae genome (formerly Nosema locustae) and compared its organization with the recently completed genome of the human parasite Encephalitozoon cuniculi. Despite being very distantly related, the genomes of these two microsporidian species have retained an unexpected degree of synteny: 13% of genes are in the same context, and 30% of the genes were separated by a small number of short rearrangements. Microsporidian genomes are, therefore, paradoxically composed of rapidly evolving sequences harbored within a slowly evolving genome, although these two processes are sometimes considered to be coupled. Microsporidian genomes show that eukaryotic genomes (like genes) do not evolve in a clock-like fashion, and genome stability may result from compaction in addition to a lack of recombination, as has been traditionally thought to occur in bacterial and organelle genomes.

Bacterial catalase in the microsporidian Nosema locustae: implications for microsporidian metabolism and genome evolution

Microsporidia constitute a group of extremely specialized intracellular parasites that infect virtually all animals. They are highly derived, reduced fungi that lack several features typical of other eukaryotes, including canonical mitochondria, flagella, and peroxisomes. Consistent with the absence of peroxisomes in microsporidia, the recently completed genome of the microsporidian Encephalitozoon cuniculi lacks a gene for catalase, the major enzymatic marker for the organelle. We show, however, that the genome of the microsporidian Nosema locustae, in contrast to that of E. cuniculi, encodes a group II large-subunit catalase. Surprisingly, phylogenetic analyses indicate that the N. locustae catalase is not specifically related to fungal homologs, as one would expect, but is instead closely related to proteobacterial sequences. This finding indicates that the N. locustae catalase is derived by lateral gene transfer from a bacterium. The catalase gene is adjacent to a large region of the genome that appears to be far less compact than is typical of microsporidian genomes, a characteristic which may make this region more amenable to the insertion of foreign genes. The N. locustae catalase gene is expressed in spores, and the protein is detectable by Western blotting. This type of catalase is a particularly robust enzyme that has been shown to function in dormant cells, indicating that the N. locustae catalase may play some functional role in the spore. There is no evidence that the N. locustae catalase functions in a cryptic peroxisome.

Congruent evidence from alpha-tubulin and beta-tubulin gene phylogenies for a zygomycete origin of microsporidia

The origin of microsporidia and the evolutionary relationships among the major lineages of fungi have been examined by molecular phylogeny using alpha-tubulin and beta-tubulin. Chytrids, basidiomycetes, ascomycetes, and microsporidia were all recovered with high support, and the zygomycetes were consistently paraphyletic. The microsporidia were found to branch within zygomycetes, and showed relationships with members of the Entomophthorales and Zoopagales. This provides support for the microsporidia having evolved from within the fungi, however, the tubulin genes are difficult to interpret unambiguously since fungal and microsporidian tubulins are very divergent. Rapid evolutionary rates a characteristic of practically all microsporidian genes studied, so determining their evolutionary history will never be completely free of such difficulties. While the tubulin phylogenies do not provide a decisive conclusion, they do further narrow the probable origin of microsporidia to a zygomycete-like ancestor.

Mitochondrial connection to the origin of the eukaryotic cell

Phylogenetic evidence is presented that primitively amitochondriate eukaryotes containing the nucleus, cytoskeleton, and endomembrane system may have never existed. Instead, the primary host for the mitochondrial progenitor may have been a chimeric prokaryote, created by fusion between an archaebacterium and a eubacterium, in which eubacterial energy metabolism (glycolysis and fermentation) was retained. A Rickettsia-like intracellular symbiont, suggested to be the last common ancestor of the family Rickettsiaceae and mitochondria, may have penetrated such a host (pro-eukaryote), surrounded by a single membrane, due to tightly membrane-associated phospholipase activity, as do present-day rickettsiae. The relatively rapid evolutionary conversion of the invader into an organelle may have occurred in a safe milieu via numerous, often dramatic, changes involving both partners, which resulted in successful coupling of the host glycolysis and the symbiont respiration. Establishment of a potent energy-generating organelle made it possible, through rapid dramatic changes, to develop genuine eukaryotic elements. Such sequential, or converging, global events could fill the gap between prokaryotes and eukaryotes known as major evolutionary discontinuity.

Microsporidia: biology and evolution of highly reduced intracellular parasites

Microsporidia are a large group of microbial eukaryotes composed exclusively of obligate intracellular parasites of other eukaryotes. Almost 150 years of microsporidian research has led to a basic understanding of many aspects of microsporidian biology, especially their unique and highly specialized mode of infection, where the parasite enters its host through a projectile tube that is expelled at high velocity. Molecular biology and genomic studies on microsporidia have also drawn attention to many other unusual features, including a unique core carbon metabolism and genomes in the size range of bacteria. These seemingly simple parasites were once thought to be the most primitive eukaryotes; however, we now know from molecular phylogeny that they are highly specialized fungi. The fungal nature of microsporidia indicates that microsporidia have undergone severe selective reduction permeating every level of their biology: From cell structures to metabolism, and from genomics to gene structure, microsporidia are reduced.

Biogenesis of iron-sulphur clusters in amitochondriate and apicomplexan protists

During the last 4 years there has been an enormous interest in the question how iron-sulphur ([Fe-S]) clusters, which are essential building blocks for life, are synthesised and assembled into apo-proteins, both in prokaryotes and in eukaryotes. The emerging picture is that the basic mechanism of this pathway has been well conserved during evolution. In yeast and probably all other eukaryotes the mitochondrion is the place where [Fe-S] clusters are synthesised, even for extramitochondrial [Fe-S] cluster-containing proteins, and a number of proteins have been functionally characterised to a certain extent within this pathway. However, almost nothing is known about this aspect in parasitic protists, although recent studies of amitochondriate protists and on the plastid-like organelle of apicomplexan parasites, the apicoplast, have started to change this. In this article I will summarise the current view of [Fe-S] cluster biogenesis in eukaryotes and discuss its implications for amitochondriate protists and for the plastid-like organelle of apicomplexan parasites.

A mitochondrial remnant in the microsporidian Trachipleistophora hominis

Microsporidia are obligate intracellular parasites of several eukaryotes. They have a highly complex and unique infection apparatus but otherwise appear structurally simple. Microsporidia are thought to lack typical eukaryotic organelles, such as mitochondria and peroxisomes. This has been interpreted as support for the hypothesis that these peculiar eukaryotes diverged before the mitochondrial endosymbiosis, which would make them one of the earliest offshoots in eukaryotic evolution. But microsporidial nuclear genes that encode orthologues of typical mitochondrial heatshock Hsp70 proteins have been detected, which provides evidence for secondary loss of the organelle or endosymbiont. In addition, gene trees and more sophisticated phylogenetic analyses have recovered microsporidia as the relatives of fungi, rather than as basal eukaryotes. Here we show that a highly specific antibody raised against a Trachipleistophora hominis Hsp70 protein detects the presence, under light and electron microscopy, of numerous tiny ( approximately 50 x 90 nm) organelles with double membranes in this human microsporidial parasite. The finding of relictual mitochondria in microsporidia provides further evidence of the reluctance of eukaryotes to lose the mitochondrial organelle, even when its canonical function of aerobic respiration has been apparently lost.