Abstract
Nucleoli were purified from the slime mold Physarum polycephalum and assayed for enrichment in ribosomal DNA by analytical ultracentrifugation. Greater than 90% of the DNA from nucleoli comprises a satellite band characteristic of Physarum ribosomal DNA (rDNA) in a CsCl equilibrium sedimentation gradient. Over 75% of the nucleolar DNA molecules are 37×106 Daltons, a further characteristic of Physarum rDNA. Nucleoli incubated with micrococcal nuclease yield a distribution of discrete DNA fragments indicative of nucleosome subunits; these digestion products are indistinguishable in size from those of total nuclear chromatin. The nucleosome DNA repeat length varied with increasing digestion from 175 down to 150 base pairs. A palindrome structure for the ribosomal nucleoprotein molecules is demonstrated by electron microscopy of actively transcribing ribosomal RNA genes, which required modification of usual methods for dispersing chromatin. In the center of each palindrome is a 6.0–6.5 μm non-transcribed “spacer” region which exhibits a beaded nucleosome structure. Transcription initiates at points on either side of the central spacer, as evidenced by 4.0–4.2 μm matrices of growing rRNA fibrils extending to each end of the palindrome. The polarity of transcription matrices, together with information about the sites of rRNA coding sequences, imply that 19S rRNA is transcribed prior to 26S rRNA.
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Grainger, R.M., Ogle, R.C. Chromatin structure of the ribosomal RNA genes in Physarum polycephalum . Chromosoma 65, 115–126 (1978). https://doi.org/10.1007/BF00329464
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DOI: https://doi.org/10.1007/BF00329464