Distribution of DNA and localization of rRNA transcription in G2 phase nucleolus of Physarum polycephalum

Using electron microscopy, NAMA-Ur DNA selective staining and BrUTP incorpo ration, the nucleolus ultrastructure, the distribution of DNA and the rRNA transcription sites in nucleolus of G2 phase Physarum polycephalum were stu died. The nucleolus was found to be different in structure from that of oth er plant cells. Fibrillar centers (FCs) were present in a large amount all over the nucleolus. DNA was distributed both in dense fibrillar components (DFC) and in FC. The DNA in the nucleolus was less condensed than that of t he chromosome territory. These changes suggested that the transcription was active within the nucleolus. BrUTP incorporation localized the rRNA transc ription in DFC and at the interface of FC and DFC, suggesting that the DNA in FC is in a storage form and only the rDNA in DFC is transcribed.