Regions in the ribosomal minichromosome of Physarum polycephalum are protected from restriction nucleases; protection is insensitive to high salt in the G phase and sensitive in the M phase of the cell cycle.

In the central spacer region of the extrachromosomal ribosomal DNA of the slime mold Physarum polycephalum, four small regions of related sequence are completely inaccessible to restriction endonucleases (HinfI and MboI). In addition, some sequences neighboring the inaccessible ones, are partially inaccessible to restriction enzymes and micrococcal nuclease. Taking advantage of the natural synchrony of Physarum plasmodia, we found that this protection is present throughout the cell cycle. Treatment with high salt (2.5 M-NaCl) of nuclei from the G2 phase of the cell cycle left the protection essentially unchanged. When nuclei from the M phase were treated with salt, the protection was abolished. The inaccessible sites are located close to the origins of replication of the rDNA.