Unusual nucleoprotein structures in the vicinity of the replication origins of the extrachromosomal rDNA of the slime mould Physarum polycephalum

Abstract

The nucleoprotein structure in the vicinity of the four replication origins of the extrachromosomal rDNA of the slime mould Physarum polycephalum was examined. At limited extents of digestion with micrococcal nuclease, a series of cleavage sites were found in the proximity of the orgins of replication. The distance between the sites reflected an altered nucleosomal organization of rDNA chromatin. It could be demonstrated that the cleavage sites were not the result of sequence-specific cutting of micrococcal nuclease. Such a different organization of rDNA chromatin was supported by recent findings that regions close to the origins of replication were inaccessible to cross-linking with psoralen. In addition, the same area contained sites that were protected from restriction endonuclease digestion (i.e., possible anchoring sites for the rDNA to the nucleolar matrix). Therefore, our results suggest that the nucleoprotein complexes presented in this paper are involved in anchoring and (or) in the replication of the rDNA.