Direct repeats surrounding the ribosomal RNA genes of Physarum polycephalum

Sequence homology was found between the external transcribed spacer and the terminal non-transcribed spacer of Physarum polycephalum rDNA. The homologous sequences were located 2kb upstream from the 19s rRNA gene and 0.3kb downstream from 26S rRNA gene, respectively, and were arranged in a direct repeat manner. Sequence analyses showed that the direct repeats consisted of two parts: one was sequences of about 130bp which showed over 90% sequence homology with each other. The other consisted mainly of many tandem repeats of a 50 to 52bp unit. The direct repeat-rRNA genes-direct repeat unit was found to be flanked by short direct repetitious sequences. Based on these findings, the significance of the direct repeat is discussed in terms of evolution of rDNA.