[Synthesis of heavy RNA during a cell cycle in Euglena gracilis]

In a previous investigation we had observed discontinuous RNA synthesis during the cell cycle of Euglena gracilis grown synchronously in the light or in the dark. In the present study, we analyse which RNA species are synthesized during the several phases of the cell cycle. During cell division, sedimentation analysis of RNA on sucrose gradients and polyacrylamide-agarose gel electrophoresis show a labeled 30 S RNA peak having a molecular weight of 2.1x10(6) daltons. This type of RNA seems heterogeneous on the basis of its behaviour in polyacrylamide-agarose gel electrophoresis, and is composed of at least two different molecular species. Their synthesis is inhibited by cycloheximide (50 μg/ml). This RNA is degraded preferentially to 13 S material. By autoradiography using a light microscope, we can observe extensive labeling in the nucleolus of dividing cells. Kinetics of labeling, chase experiments, and concomitant synthesis of heavier RNA (40 S and 35 S) tend to identify the 30 S RNA as the immediate precursor of one or both cytoplasmic ribosomal RNA molecules.