Nucleotide sequence of the central non-transcribed spacer region of Physarum polycephalum rDNA.

In Physarum polycephalum the rRNA genes are present on linear extrachromosomal molecules, each containing two transcription units arranged as a giant palindrome. In the center of the molecule, between the two transcription units, is the 23-kb central spacer, previously shown to contain the replication origins and several regions of direct and inverted repeats. Segments of all the repeats in the spacer have been sequenced and their overall organization determined. The entire spacer consists of reiterations of only 1200 bp of different DNA sequences. The sequence surrounding the transcription start point is not repeated in the spacer, as it is in Xenopus and Drosophila. Sequencing of purified, uncloned rDNA localized some of the methylcytosine residues in the spacer. The repetitious sequences appear to be undergoing concerted evolution.