Diatom genomics: genetic acquisitions and mergers.
(1/58)
Diatom algae arose by two-step endosymbiosis. The complete genome of the diatom Thalassiosira pseudonana has now been sequenced, allowing us to reconstruct the remarkable intracellular gene transfers that occurred during this convoluted cellular evolution. (+info)
On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes.
(2/58)
A global phylogeny of major eukaryotic lineages is a significant and ongoing challenge to molecular phylogenetics. Currently, there are five hypothesized major lineages or 'supergroups' of eukaryotes. One of these, the chromalveolates, represents a large fraction of protist and algal diversity. The chromalveolate hypothesis was originally based on similarities between the photosynthetic organelles (plastids) found in many of its members and has been supported by analyses of plastid-related genes. However, since plastids can move between eukaryotic lineages, it is important to provide additional support from data generated from the nuclear-cytosolic host lineage. Genes coding for six different cytosolic proteins from a variety of chromalveolates (yielding 68 new gene sequences) have been characterized so that multiple gene analyses, including all six major lineages of chromalveolates, could be compared and concatenated with data representing all five hypothesized supergroups. Overall support for much of the phylogenies is decreased over previous analyses that concatenated fewer genes for fewer taxa. Nevertheless, four of the six chromalveolate lineages (apicomplexans, ciliates, dinoflagellates and heterokonts) consistently form a monophyletic assemblage, whereas the remaining two (cryptomonads and haptophytes) form a weakly supported group. Whereas these results are consistent with the monophyly of chromalveolates inferred from plastid data, testing this hypothesis is going to require a substantial increase in data from a wide variety of organisms. (+info)
Unique genetic compartmentalization of the SUF system in cryptophytes and characterization of a SufD mutant in Arabidopsis thaliana.
(3/58)
The mobilization of sulfur (SUF) system is one of three systems involved in iron-sulfur cluster biosynthesis and maintenance. In eukaryotes the SUF system is specific for the plastid and therefore of symbiotic origin. Analyses in cryptophytes showed a unique genetic compartmentalization of the SUF system, which evolved by at least two different gene transfer events. We analyzed one of the components, SufD, in the cryptophyte Guillardia theta and in Arabidopsis thaliana. We demonstrated that SufD fulfils house keeping functions during embryogenesis and in adult plants in A. thaliana. (+info)
Unique tRNA introns of an enslaved algal cell.
(4/58)
Nucleomorphs are remnant nuclei of eukaryotic, secondary endosymbionts exclusively found in cryptophytes and chlorarachniophytes. The nucleomorph of the cryptophyte Guillardia theta codes for 36 transfer RNA (tRNA) genes, 15 of them predicted to contain introns and 1 pseudo-tRNA. Some of the predicted intervening sequences are manifested at positions not known in Eukarya, even tRNAs with more than one intron were suggested. By isolating reverse transcriptase-polymerase chain reaction products of the spliced tRNAs we verify the processing of all predicted intron-harboring tRNAs and demonstrate the splicing of the smallest introns (3 nt) investigated so far. However, the spliced intervening sequences are in some cases shifted in respect to the predicted ones. Moreover, we show that introns, if inserted into the B-box of tRNA genes in the nucleomorph of cryptophytes, mimic promoter regions and do not abolish transcription by RNA polymerase III. Consequently, internal nucleomorph-encoded tRNA promoter regions are in some cases dissected from the sequence of the mature tRNAs. By reanalyzing tRNA introns of a recently sequenced red algae we furthermore show that splicing of introns at unusual positions may be introduced in cryptophytes by its secondary endosymbiont. However, in contrast to the rest of the symbiont genome, introns are not minimized in quantity but are instead scattered along the tRNA genes. (+info)
A high frequency of overlapping gene expression in compacted eukaryotic genomes.
(5/58)
The gene density of eukaryotic nuclear genomes is generally low relative to prokaryotes, but several eukaryotic lineages (many parasites or endosymbionts) have independently evolved highly compacted, gene-dense genomes. The best studied of these are the microsporidia, highly adapted fungal parasites, and the nucleomorphs, relict nuclei of endosymbiotic algae found in cryptomonads and chlorarachniophytes. These systems are now models for the effects of compaction on the form and dynamics of the nuclear genome. Here we report a large-scale investigation of gene expression from compacted eukaryotic genomes. We have conducted EST surveys of the microsporidian Antonospora locustae and nucleomorphs of the cryptomonad Guillardia theta and the chlorarachniophyte Bigelowiella natans. In all three systems we find a high frequency of mRNA molecules that encode sequence from more than one gene. There is no bias for these genes to be on the same strand, so it is unlikely that these mRNAs represent operons. Instead, compaction appears to have reduced the intergenic regions to such an extent that control elements like promoters and terminators have been forced into or beyond adjacent genes, resulting in long untranslated regions that encode other genes. Normally, transcriptional overlap can interfere with expression of a gene, but these genomes cope with high frequencies of overlap and with termination signals within expressed genes. These findings also point to serious practical difficulties in studying expression in compacted genomes, because many techniques, such as arrays or serial analysis of gene expression will be misleading. (+info)
The Rhodomonas salina mitochondrial genome: bacteria-like operons, compact gene arrangement and complex repeat region.
(6/58)
To gain insight into the mitochondrial genome structure and gene content of a putatively ancestral group of eukaryotes, the cryptophytes, we sequenced the complete mitochondrial DNA of Rhodomonas salina. The 48 063 bp circular-mapping molecule codes for 2 rRNAs, 27 tRNAs and 40 proteins including 23 components of oxidative phosphorylation, 15 ribosomal proteins and two subunits of tat translocase. One potential protein (ORF161) is without assigned function. Only two introns occur in the genome; both are present within cox1 belong to group II and contain RT open reading frames. Primitive genome features include bacteria-like rRNAs and tRNAs, ribosomal protein genes organized in large clusters resembling bacterial operons and the presence of the otherwise rare genes such as rps1 and tatA. The highly compact gene organization contrasts with the presence of a 4.7 kb long, repeat-containing intergenic region. Repeat motifs approximately 40-700 bp long occur up to 31 times, forming a complex repeat structure. Tandem repeats are the major arrangement but the region also includes a large, approximately 3 kb, inverted repeat and several potentially stable approximately 40-80 bp long hairpin structures. We provide evidence that the large repeat region is involved in replication and transcription initiation, predict a promoter motif that occurs in three locations and discuss two likely scenarios of how this highly structured repeat region might have evolved. (+info)
Rhodopsin-mediated photoreception in cryptophyte flagellates.
(7/58)
We show that phototaxis in cryptophytes is likely mediated by a two-rhodopsin-based photosensory mechanism similar to that recently demonstrated in the green alga Chlamydomonas reinhardtii, and for the first time, to our knowledge, report spectroscopic and charge movement properties of cryptophyte algal rhodopsins. The marine cryptophyte Guillardia theta exhibits positive phototaxis with maximum sensitivity at 450 nm and a secondary band above 500 nm. Variability of the relative sensitivities at these wavelengths and light-dependent inhibition of phototaxis in both bands by hydroxylamine suggest the involvement of two rhodopsin photoreceptors. In the related freshwater cryptophyte Cryptomonas sp. two photoreceptor currents similar to those mediated by the two sensory rhodopsins in green algae were recorded. Two cDNA sequences from G. theta and one from Cryptomonas encoding proteins homologous to type 1 opsins were identified. The photochemical reaction cycle of one Escherichia-coli-expressed rhodopsin from G. theta (GtR1) involves K-, M-, and O-like intermediates with relatively slow (approximately 80 ms) turnover time. GtR1 shows lack of light-driven proton pumping activity in E. coli cells, although carboxylated residues are at the positions of the Schiff base proton acceptor and donor as in proton pumping rhodopsins. The absorption spectrum, corresponding to the long-wavelength band of phototaxis sensitivity, makes this pigment a candidate for one of the G. theta sensory rhodopsins. A second rhodopsin from G. theta (GtR2) and the one from Cryptomonas have noncarboxylated residues at the donor position as in known sensory rhodopsins. (+info)
Lineage-specific variations of congruent evolution among DNA sequences from three genomes, and relaxed selective constraints on rbcL in Cryptomonas (Cryptophyceae).
(8/58)
BACKGROUND: Plastid-bearing cryptophytes like Cryptomonas contain four genomes in a cell, the nucleus, the nucleomorph, the plastid genome and the mitochondrial genome. Comparative phylogenetic analyses encompassing DNA sequences from three different genomes were performed on nineteen photosynthetic and four colorless Cryptomonas strains. Twenty-three rbcL genes and fourteen nuclear SSU rDNA sequences were newly sequenced to examine the impact of photosynthesis loss on codon usage in the rbcL genes, and to compare the rbcL gene phylogeny in terms of tree topology and evolutionary rates with phylogenies inferred from nuclear ribosomal DNA (concatenated SSU rDNA, ITS2 and partial LSU rDNA), and nucleomorph SSU rDNA. RESULTS: Largely congruent branching patterns and accelerated evolutionary rates were found in nucleomorph SSU rDNA and rbcL genes in a clade that consisted of photosynthetic and colorless species suggesting a coevolution of the two genomes. The extremely accelerated rates in the rbcL phylogeny correlated with a shift from selection to mutation drift in codon usage of two-fold degenerate NNY codons comprising the amino acids asparagine, aspartate, histidine, phenylalanine, and tyrosine. Cysteine was the sole exception. The shift in codon usage seemed to follow a gradient from early diverging photosynthetic to late diverging photosynthetic or heterotrophic taxa along the branches. In the early branching taxa, codon preferences were changed in one to two amino acids, whereas in the late diverging taxa, including the colorless strains, between four and five amino acids showed changes in codon usage. CONCLUSION: Nucleomorph and plastid gene phylogenies indicate that loss of photosynthesis in the colorless Cryptomonas strains examined in this study possibly was the result of accelerated evolutionary rates that started already in photosynthetic ancestors. Shifts in codon usage are usually considered to be caused by changes in functional constraints and in gene expression levels. Thus, the increasing influence of mutation drift on codon usage along the clade may indicate gradually relaxed constraints and reduced expression levels on the rbcL gene, finally correlating with a loss of photosynthesis in the colorless Cryptomonas paramaecium strains. (+info)