Dinoflagellates are unicellular primitive eukaryotes, as shown by ultrastructural, biochemical and molecular studies. Most are phytoplanktonic and are considered to be an important link in the trophic chain. Several species are able to produce dangerous toxins and/or to anarchically proliferate in their aquatic environment. These cells are distinguished by features of their chromatin, nuclear apparatus and mitotic system. From a molecular point of view, they are the only known eukaryotes in which the chromatin is totally devoid of histones and nucleosomes. Their chromosomal nucleofilaments are organized into a right-handed double helical bundle in a hierarchy of 6 levels of organization, this architecture being maintained by divalent cations and structural RNAs. Their chromosomes are permanently compacted in the nuclear envelope which persists throughout the typically eukaryotic cell cycle. Their genome is composed of an enormous quantity of DNA (1 to 10 times the human genome and 200 to 2 000 times the Saccharomyces genome) and its base composition is unique. Previous work using 3H adenine incorporation showed that RNA transcription occurs on extrachromosomal nucleofilaments. By means of electron microscopy (EM)and immunocytochemistry, we recently demonstrated that genetically active DNA is located in the periphery of the chromosomes where Z-DNA sequences and a specific basic nuclear protein (HCc, p14) are located, the main body being composed of highly repeated sequences in a B-DNA conformation. Enzymatic digestions of genomic Crypthecodinium cohnii DNA followed by hybridizations either with a cDNA or an rDNA gene also led to the conclusions that active chromatin is peripherally located. In the nucleolus, both B- and Z-DNA as well as the HCc protein are found in the nucleolar organizingregion (NOR) while the rRNA coding sequences were detected by in situ hybridization in the periphery of the NOR where EM shows that chromosome nucleofilaments are partially unwound. So, a local unwinding of the peripheral loop nucleofilaments, favored by the presence of Z-DNA sequences, and of the topoisomerase II homologue appears essential for the completion of the transcription process.
Chromosomes, Nucleus, Nucleolus, rRNA genes, Dinoflagellates
