High levels of transition metals in dinoflagellate chromosomes

Distinctive Nuclear Features of Dinoflagellates with A Particular Focus on Histone and Histone-Replacement Proteins

Dinoflagellates are important eukaryotic microorganisms that play critical roles as producers and grazers, and cause harmful algal blooms. The unusual nuclei of dinoflagellates "dinokaryon" have led researchers to investigate their enigmatic nuclear features. Their nuclei are unusual in terms of their permanently condensed nucleosome-less chromatin, immense genome, low protein to DNA ratio, guanine-cytosine rich methylated DNA, and unique mitosis process. Furthermore, dinoflagellates are the only known group of eukaryotes that apparently lack histone proteins. Over the course of evolution, dinoflagellates have recruited other proteins, e.g., histone-like proteins (HLPs), from bacteria and dinoflagellates/viral nucleoproteins (DVNPs) from viruses as histone substitutes. Expression diversity of these nucleoproteins has greatly influenced the chromatin structure and gene expression regulation in dinoflagellates. Histone replacement proteins (HLPs and DVNPs) are hypothesized to perform a few similar roles as histone proteins do in other eukaryotes, i.e., gene expression regulation and repairing DNA. However, their role in bulk packaging of DNA is not significant as low amounts of proteins are associated with the gigantic genome. This review intends to summarize the discoveries encompassing unique nuclear features of dinoflagellates, particularly focusing on histone and histone replacement proteins. In addition, a comprehensive view of the evolution of dinoflagellate nuclei is presented.

Structural DNA and genetically active DNA in dinoflagellate chromosomes

High resolution electron microscope autoradiographs of [3H]adenine incorporation in the dinoflagellate Prorocentrum micans suggest that RNA transcription occurs on extrachromosomal DNA filaments, but not on DNA in the main body of the chromosome. This genetically inactive DNA has an important role, however, in stabilising chromosome structure by its association with protein matrix. Evidence for the importance of this molecular association (which is probably cation-mediated) is provided by alkaline buffer extraction of the protein matrix in chromosomes of Amphidinium carterae , leading to complete destabilisation of the DNA framework. The clear distinction between structural DNA and genetically active DNA in these chromosomes provides a marked contrast to normal eucaryote chromosomes. This distinction is related to the occurrence of high DNA values in these organisms, and the evolutionary status of the dinoflagellate chromosome.

X-ray microanalysis of unfixed chromatin in dinoflagellate cells prepared by a monolayer cryotechnique

The fine structural appearance and elemental composition of unfixed chromatin is described for cells of the dinoflagellate Prorocentrum micans prepared by a freeze-drying technique. This involves rapid freezing, cryo-dehydration and resin infiltration of a monolayer of cells dispersed over renin base. The fine structure of the nucleus appears quite different from chemically fixed and dehydrated cells. In stained sections, the chromosomes are seen as pale areas containing diffuse chromatin, and are surrounded by electron-dense nucleoplasm which has a reticulate substructure. X-ray microanalysis reveals the presence of high levels of chromatin-associated Ca and transition metals Fe, Ni, Cu and Zn, in accordance with previous observations on chemically processed cells. Calculation of elemental mass fractions by on-line computer demonstrates an overall ratio of one divalent cation per two phosphorus groups (or per two nucleotides). This ratio is similar to that obtained previously for chemically fixed chromatin, and shows that the precise overall association of metals is not primarily determined by the process of fixation.

Levels of dinoflagellate chromosome-bound metals in conditions of low external ion availability: an x-ray microanalytical study

Cells of the dinoflagellate Glenodinium foliaceum were able to grow and divide in artificial (AE50) liquid culture medium diluted down to 10% of its normal concentration. X-ray microanalysis, with on-line computation of mass fractions, was used to determine the quantitative occurrence of bound (insoluble) elements in the chromosomes of cells grown in normal, 30 and 10% medium. In normal medium, chromosomes contained high levels of phosphorus (nucleic acid), sulphur (protein) and a variety of bound divalent metals, including Ca, Fe, Ni, Cu and Zn. With a decrease in the level of external ions (30 and 10% dilutions), the level of sulphur showed a significant decrease, but the mass fractions of insoluble phosphorus and total divalent metals did not fall. The presence of these metals in dinoflagellate chromatin does not, therefore, simply involve intracellular deposition in response to high external levels. The occurrence of divalent metals showed variation with medium concentration in terms of the proportions of different metals present, the nature of the metal--nucleoprotein associations, the metal concentrations per unit mass of chromatin and the number of metal atoms per 100 nucleotides. None of the above characteristics is therefore constant in an individual species.