Synthesis and Processing of Ribosomal RNA in Cultured Plant Cells

Four species of rapidly labelled, high‐molecular‐weight RNA have been extracted from partially purified nuclei of cultured sycamore cells. The molecular weights of these RNA species have been tentatively estimated by comparison of their electrophoretic mobilities in polyacrylamide gels with thóse of HeLa 45‐S and 32‐S RNA as 3.4 × 106, 2.4 × 106, 1.4 × 106 and 1.0 × 106. All four species of RNA are methylated and the components with molecular weights of 2.4 × 106, 1.4 × 106 and 1.0 × 106 have a pattern of labelling consistent with their being precursors of ribosomal RNA. Labelling studies with [14C]uridine and [Me‐3H]methionine have indicated that the 2.4 × 106‐Mr molecule is cleaved to give the 1.4 × 106‐Mr molecule and the 1.0 × 106‐Mr molecule which then give rise to 1.3 × 106‐Mr and 0.7 × 106‐Mr ribosomal RNAs respectively. The kinetics of labelling of the 3.4 × 106‐Mr component, however, show that it cannot be an obligatory precursor of the 2.4 × 106‐Mr RNA and the implications of this observation for the process of transcription of the ribosomal genes and the method of cleavage of the transcription products are discussed. The kinetics of labelling of the RNA with [Me‐3H]methionine have indicated that methylation occurs during the synthesis of the largest precursor molecules, as is known to be the case in mammalian tissues but, in contrast to the mammalian system, there also appears to be a significant amount of direct methylation of the 1.4 × 106‐Mr precursor RNA.