PROPERTIES OF RIBOSOMAL RNA PRECURSOR IN XENOPUS LAEVIS - COMPARISON TO PRECURSOR IN MAMMALS AND IN PLANTS

Polyacrylamide gel electrophoresis was used to analyse the rapidly labelled RNA in Xenopus laevis cultured kidney cells. The ribosomal precursor was identified by its base composition and found to have a molecular weight of 2.5 to 2.6 × 106. This is 0.3 to 0.4 × 106 greater than the sum of the weights of the ribosomal RNA. This excess of non-ribosomal RNA has a high content of G + C and is assumed to be lost during processing. The precursor in plant tissues was shown to be similar to that in Xenopus. A relatively long-lived intermediate in the processing was found in Xenopus and plants, which had a molecular weight of 0.1 × 106 greater than the heavy ribosomal RNA; it was assumed to be a precursor to the latter. These amounts of excess non-ribosomal RNA are much smaller than in the mammalian 45 and 32 s precursors. It is concluded that the very high molecular weight precursors which contain about 40% of excess RNA are peculiar to the mammals. The results are correlated with other work on the ribosomal DNA and on the structures of the nucleolar core. © 1969.