Organization of the ribosomal RNA genes of Dictyostelium discoideum. Mapping of the transcribed region

We mapped the transcribed region of the ribosomal RNA genes and determined the scheme for processing of the 36S rRNA precursor. Four of the repeated fragments seen in an EcoRI digestion of nuclear DNA code for rRNA: bands V, VII, IXa and IXb. The 17 S rRNA gene was shown by hybridization to recombinant plasmids (rRNA genes, pMB9) to be located in bands V and VII. Similarly, the 26 S rRNA gene was shown to cover the other end of band VII and all of bands IXa and IXb. Electron microscope mapping, using R-loops and the T4 gene 32 protein procedure, showed that the transcribed spacer between the 17 S and 26 S genes is 1 kilobase long. The 5·8 S rRNA gene is located within 200 nucleotides of the 26 S rRNA gene in the transcribed spacer region. We also derived a processing scheme for the rRNA including mapping of the external transcribed spacer. The 36 S initial transcript is split into the 28 S and 21 S molecules. The 28 S nuclear rRNA gives rise to the 26 S and 5·8 S species and contains little transcribed spacer. The 21 S is processed to the 17 S mature species, with some of the transcribed spacer at the 5′ end of the transcript. The 17 S species was found to be at the 5′ end of the 36 S precursor and the 26 S rRNA was located at the 3′ end.