CYTOCHEMICAL AND AUTORADIOGRAPHIC STUDY OF THE NUCLEUS OF A SYMBIOTICDINOFLAGELLATE

Non-nucleolar ribonucleoprotein (RNP) particles, extended chromatin, a nd the distribution of very active sites of transcription are studied in a dinoflagellate symbiont of sea anemones. A preferential staining method for RNP demonstrates granules 47 +/- 10 nm (standard deviation) in diameter, partially surrounded by a clear halo, and fibres of diff erent thickness. All these particles are sensitive to RNase. Phosphotu ngstic acid staining for chromatin shows loops of extended chromatin f ormed by filaments about 7.5 +/- 3 nm thick protruding from chromosome s. These filaments are sensitive to DNase. Short labelings with tritia ted uridine demonstrate a thin perichromosomal region of high transcri ptional activity, which may be the functional equivalent to the perich romatin region of higher eukaryotes. In contrast to the eukaryotic per ichromatin region, the perichromosomal zone of dinoflagellates has no feature allowing its morphological characterization. A network of fila ments 4 +/- 2nm thick, resistant to RNase and to DNase, may be related to the nuclear matrix. A sheath of RNP nature is described at the per iphery of chromosomes. It is suggested that this sheath is related to the ordered disposition of chromosomal fibres.