Genome sequence and gene compaction of the eukaryote parasite Encephalitozoon cuniculi.

Microsporidia are obligate intracellular parasites infesting many animal groups. Lacking mitochondria and peroxysomes, these unicellular eukaryotes were first considered a deeply branching protist lineage that diverged before the endosymbiotic event that led to mitochondria. The discovery of a gene for a mitochondrial-type chaperone combined with molecular phylogenetic data later implied that microsporidia are atypical fungi that lost mitochondria during evolution. Here we report the DNA sequences of the 11 chromosomes of the approximately 2.9-megabase (Mb) genome of Encephalitozoon cuniculi (1,997 potential protein-coding genes). Genome compaction is reflected by reduced intergenic spacers and by the shortness of most putative proteins relative to their eukaryote orthologues. The strong host dependence is illustrated by the lack of genes for some biosynthetic pathways and for the tricarboxylic acid cycle. Phylogenetic analysis lends substantial credit to the fungal affiliation of microsporidia. Because the E. cuniculi genome contains genes related to some mitochondrial functions (for example, Fe-S cluster assembly), we hypothesize that microsporidia have retained a mitochondrion-derived organelle.

Sequence and analysis of chromosome I of the amitochondriate intracellular parasite Encephalitozoon cuniculi (Microspora).

A DNA sequencing program was applied to the small (<3 Mb) genome of the microsporidian Encephalitozoon cuniculi, an amitochondriate eukaryotic parasite of mammals, and the sequence of the smallest chromosome was determined. The approximately 224-kb E. cuniculi chromosome I exhibits a dyad symmetry characterized by two identical 37-kb subtelomeric regions which are divergently oriented and extend just downstream of the inverted copies of an 8-kb duplicated cluster of six genes. Each subtelomeric region comprises a single 16S-23S rDNA transcription unit, flanked by various tandemly repeated sequences, and ends with approximately 1 kb of heterogeneous telomeric repeats. The central (or core) region of the chromosome harbors a highly compact arrangement of 132 potential protein-coding genes plus two tRNA genes (one gene per 1.14 kb). Most genes occur as single copies with no identified introns. Of these putative genes, only 53 could be assigned to known functions. A number of genes from the transcription and translation machineries as well as from other cellular processes display characteristic eukaryotic signatures or are clearly eukaryote-specific.

Fibroblast growth factor receptors participate in the control of mitogen-activated protein kinase activity during nerve growth factor-induced neuronal differentiation of PC12 cells.

The current paradigm for the role of nerve growth factor (NGF) or FGF-2 in the differentiation of neuronal cells implies their binding to specific receptors and activation of kinase cascades leading to the expression of differentiation specific genes. We examined herein the hypothesis that FGF receptors (FGFRs) are involved in NGF-induced neuritogenesis of pheochromocytoma-derived PC12 cells. We demonstrate that in PC12 cells, FGFR expression and activity are modulated upon NGF treatment and that a dominant negative FGFR-2 reduces NGF-induced neuritogenesis. Moreover, FGF-2 expression is modulated by NGF, and FGF-2 is detected at the cell surface. Oligonucleotides that specifically inhibit FGF-2 binding to its receptors are able to significantly reduce NGF-induced neurite outgrowth. Finally, the duration of mitogen-activated protein kinase (MAPK) activity upon FGF or NGF stimulation is shortened in FGFR-2 dominant negative cells through inactivation of signaling from the receptor to the Ras/MAPK pathway. In conclusion, these results demonstrate that FGFR activation is involved in neuritogenesis induced by NGF where it contributes to a sustained MAPK activity in response to NGF.

Identification and characterization of an intracellular protein complex that binds fibroblast growth factor-2 in bovine brain.

The fibroblast growth factor (FGF) family is composed of polypeptides with sequence identity which signal through transmembrane tyrosine kinase receptors. We report here the purification from bovine brain microsomes of an FGF-2-binding complex composed of three proteins of apparent molecular masses 150 kDa, 79 kDa and 46 kDa. Only the 150 kDa and 79 kDa proteins bound FGF-2 in cross-linking and ligand-blotting experiments. Binding of FGF-2 to p79 is enhanced in the presence of calcium. Peptide sequences allowed the identification of p150 and the cloning of the cDNAs encoding p79 and p46. The deduced amino acid sequence of p79 reveals high similarity to those of gastrin-binding protein and mitochondrial enoyl-CoA hydratase/hydroxyacyl-CoA dehydrogenase. p46 is similar to mitochondrial ketoacyl-CoA thiolase. Stable transfection of FR3T3 rat fibroblast cells with p79 cDNA analysed by electron microscopy following immunolabelling of ultra-thin cryosections revealed a localization of p79 in the secretory pathway, mainly in the endoplasmic reticulum and the Golgi region, where it is specifically associated with the molecular chaperone calnexin. In vivo a protein similar to the Golgi protein MG-160 forms a complex with FGF-2 and p79.

Interstitial telomeres are hotspots for illegitimate recombination with DNA molecules injected into the macronucleus of Paramecium primaurelia.

DNA molecules injected into the macronucleus of Paramecium primaurelia replicate either as free linear telomerized or chromosome integrated molecules. In the present study we show that when a 1.77 kb BamHI DNA fragment harbouring the his3 gene of Saccharomyces cerevisiae was microinjected into the macronucleus, a fraction of the molecules are integrated into the chromosome via an illegitimate recombination process. The injected molecules were mostly inserted at their extremities at multiple points in the genome by replacing the Paramecium sequences. However, insertion sites were not totally at random. Roughly 30% of the molecules were integrated next to or in telomeric repeats. These telomeric repeats were not at the extremities of chromosomes but occupy an internal or interstitial position. We argue that such sites are hotspots for integration as the probability of random insertion near or in an interstitial telomeric site, of which there are 25-60 in a macronucleus is between 5 x 10(-4) and 3 x 10(-5).

Telomeres inhibit end to end fusion and enhance maintenance of linear DNA molecules injected into the Paramecium primaurelia macronucleus.

Direct injection into the macronucleus of Paramecium tetraurelia of DNA molecules coding for the A-antigen leads to expression of the gene and autonomous replication. When injected into Paramecium primaurelia DNA from probably any origin, procaryote or eucaryote, can replicate as linear telomerized molecules and the number of copies maintained can be very high (up to 20000 copies). We present here evidence that if the injected linear DNA molecules harbour preexisting telomeres at both extremities they are protected from degradation, the number of DNA molecules maintained being 15- to 30-fold higher than if the molecules are injected without telomeres. Some of the injected molecules replicate as multimers, but, only when the fused ends are devoid of preexisting telomeric repeats.

RNA-dependent DNA polymerase activity in Paramecium tetraurelia: what for?

Protein extracts from the protozoan ciliate Paramecium tetraurelia revealed high levels of RNA-dependent DNA polymerase activity (reverse transcriptase). Stable and constant during the somatic phase of the cell cycle, the reverse transcriptase activity quickly diminished following the completion of the sexual phases of the cell cycle: conjugation and autogamy. The Paramecium reverse transcriptase presented a number of common features with retroviral polymerases: ability to copy synthetic templates such as poly(rCm).oligo(dG) as well as mRNA; sensitivity to various reverse transcriptase inhibitors such as HPA 23, suramin, phosphonoformate and ethidium bromide; insensitivity to the action of other DNA and RNA polymerase inhibitors and, finally, the requirement for divalent cations before the enzyme can function: either magnesium or manganese. Although the reverse transcriptase activity was not proven to be independent from one of the DNA polymerases in paramecia, its high activity predicts a role in the paramecia cell cycle. From what we are able to conceive today two possible roles could be envisaged. Participation in the anlage macronucleus formation: micronuclear sequences are first transcripted and, after rearrangements of the RNA molecules, these are retrotranscribed into the macronuclear DNA molecules or association with retrotransposons that participate in the movement of certain macronuclear sequences into the germ-line micronucleus.