A Gene in the Process of Endosymbiotic Transfer

BACKGROUND: The endosymbiotic birth of organelles is accompanied by massive transfer of endosymbiont genes to the eukaryotic host nucleus. In the centric diatom Thalassiosira pseudonana the Psb28 protein is encoded in the plastid genome while a second version is nuclear-encoded and possesses a bipartite N-terminal presequence necessary to target the protein into the diatom complex plastid. Thus it can represent a gene captured during endosymbiotic gene transfer. METHODOLOGY/PRINCIPAL FINDINGS: To specify the origin of nuclear- and plastid-encoded Psb28 in T. pseudonana we have performed extensive phylogenetic analyses of both mentioned genes. We have also experimentally tested the intracellular location of the nuclear-encoded Psb28 protein (nuPsb28) through transformation of the diatom Phaeodactylum tricornutum with the gene in question fused to EYFP. CONCLUSIONS/SIGNIFICANCE: We show here that both versions of the psb28 gene in T. pseudonana are transcribed. We also provide experimental evidence for successful targeting of the nuPsb28 fused with EYFP to the diatom complex plastid. Extensive phylogenetic analyses demonstrate that nucleotide composition of the analyzed genes deeply influences the tree topology and that appropriate methods designed to deal with a compositional bias of the sequences and the long branch attraction artefact (LBA) need to be used to overcome this obstacle. We propose that nuclear psb28 in T. pseudonana is a duplicate of a plastid localized version, and that it has been transferred from its endosymbiont

The mitochondrial genome of the brown alga Laminaria digitata: a comparative analysis

We report here the complete sequence of the mitochondrial genome of the brown alga Laminaria digitata (Hudson) J.V. Lamouroux. L. digitata mtDNA is a circular molecule of 38,007 bp (64.9 % A+ T), encoding 63 genes and 3 ORFs and with only 6-7 % of non-coding sequences. Based on gene content and order, its overall organization is very similar to that of the mitochondrial genome of Pylaiella littoralis, another brown alga belonging to a different sublineage of the Phaeophyceae. In particular, the two genomes share unusual features, which hence could be unique to brown algae among the heterokont lineage, namely the presence of a rn5 gene, a short nad I I gene, a cox2 gene with a large in-frame insertion and alpha-proteobacterial-like promoter sequences. On the other hand, L. digitata lacks the sequences which are thought to have been transmitted horizontally to the P. littoralis genome, that is, the group-II introns in the rnl and cox1 genes, and it features only traces of an ancestral T7-like RNA polymerase. Distance phylogenetic trees inferred from concatenated mitochondrial genes confirm that speciation of brown algae occurred recently compared to other heterokonts

A mtDNA-based phylogeny of the brown algal genus Fucus (Heterokontophyta; Phaeophyta)

Species of Fucus are among the dominant seaweeds along Northern Hemisphere shores, but taxonomic designations often are confounded by significant intraspecific morphological variability. We analyzed intra- and inter-specific phylogenetic relationships within the genus (275 individuals representing 16 taxa) using two regions of the mitochondrion: a variable intergenic spacer and a conserved portion of the 23S subunit. Bayesian ML and MP analyses verified a shallow phylogeny with two major lineages (previously reported) and resolved some intra-lineage relationships. Significant species-level paraphyly/polyphyly was observed within lineages 1A and 2. Despite higher species richness in the North Atlantic, a North Pacific origin of the genus is supported by a gradient of decreasing haplotype and nucleotide diversities in F. distichus from the North Pacific to the East Atlantic

Carbon fixation in diatoms

Diatoms are unicellular photoautotrophic algae and very successful primary producers in the oceans. Their high primary productivity is probably sustained by their high adaptability and a uniquely arranged metabolism. Diatom belongs to the Chromista, a large eukaryotic group, which has evolved by multiple endosymbiotic steps. As a result, diatoms possess a plastids with four membranes together with complicated translocation systems to transport proteins and metabolites including inorganic substances into and out of the plastids. In addition to the occurrence of potential plasma-membrane transporters, there are numerous carbonic anhydrases (CAs) within the matrix of the layered plastidic membranes, strongly suggesting large interconversion activity between CO2 and HCO3 − within the chloroplast envelope as a part of a CO2-concentrating mechanism (CCM). In diatoms also the Calvin cycle and its adjacent metabolism reveal unique characteristics as, for instance, ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) activase, the plastidic sedoheptulose-1,7-bisphosphatase (SBPase), and the plastidic oxidative pentose phosphate pathway (OPP) are absent. Furthermore, the Calvin cycle metabolism in diatoms is not under the strict redox control by the thioredoxin (Trx) system. Instead, a CO2-supplying system in the pyrenoid shows CA activities which are probably regulated by chloroplastic Trxs. Pyrenoidal CAs are also regulated on the transcriptional level by CO2 concentrations via cAMP as a second messenger, suggesting an intense control system of CO2 acquisition in response to CO2 availability. The photorespiratory carbon oxidation cycle (PCOC) is the major pathway to recycle phosphoglycolate in diatoms although this process might not be involved in recycling of 3-phosphoglycerate but instead produces glycine and serine. In this review we focus on recent experimental data together with supportive genome information of CO2 acquisition and fixation systems primarily in two marine diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana