Unicellular Dinoflagellates represent the only eukaryotic Phylum lacking histones and nucleosomes. To investigate whether Dinoflagellates do have a nuclear matrix that would modulate the supramolecular organization of their non-nucleosomal DNA and chromosomes, cells of the free-living unarmored Dinoflagellate Amphidinium carterae were encapsulated in agarose microbeads and submitted to sequential extraction with non-ionic detergents, nucleases and 2 M NaCl. Our results demonstrate that this species has a residual nuclear matrix similar to that of vertebrates and higher plants. The cytoskeleton-nuclear matrix complex of A. carterae shows a relatively intricate polypeptide pattern. Immunoblots with different antibodies reveal several intermediate filament types of proteins, one of which is immunologically related to vertebrate lamins, confirming that these proteins are ancestral members of the IF family, which is highly conserved in eukaryotes. A topoisomerase II homologue has also been identified in the nuclear matrix, suggesting that these structures could play a role in organizing the Dinoflagellate DNA in loop domains. Taken together our results demonstrate that the nuclear matrix is an early acquisition of the eukaryotic nucleus, independent of histones and nucleosomes in such a way that the mechanisms controlling the two levels of organization in eukaryotic chromatin would be molecularly and evolutionarily independent.

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