Certain features of the dinoflagellate nucleus suggest that it represents a primitive form of eukaryotic nucleus. For this reason, it was of interest to characterize dinoflagellate ribosomal RNA (rRNA) and its mode of synthesis to determine if it also deviated from typical eukaryotic patterns. Gyrodinium cohnii was chosen for this examination. Gyrodinium ribosomal RNA species are 16 and 25s as judged by their sedimentation velocities in isokinetic sucrose gradients. These values are typical of higher plants. In addition, the RNA cosedimented precisely with rRNA from the ciliate Tetrahymena. Nucleotide ratio analyses revealed a GMP + CMP content of 46% for both species of rRNA. The kinetics of incorporation of a radioactive precursor into ribosomal RNA have also been studied, and it seems likely that the maturation of rRNA starts with the synthesis of a 38s molecule. This serves as precursor to the 16s species, and, after a 27s intermediate, the 25s ribosomal component. The process is similar to that in other eukaryotes. The structure of the nucleolus has also been examined, and is seen to be typically eukaryotic.

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