Ultrastructural localization of transcription sites, DNA, and RNA reveals a concentric arrangement of structural and functional domains in plant nucleolonema

By light microscopy the filamentous structure called nucleolonema was visualized in the nucleolus ofAllium cepa. Sites of rDNA transcription were located in the nucleolonema at the electron microscopic level. Thin sections of root tips were treated for desired times with bromo-UTP (BrUTP) and the sites of BrUTP incorporation were determined by immunogold labeling with antibromodeoxyuridine antibodies. The label was mostly seen at the periphery of the fibrillar centers and in the dense fibrillar component immediately surrounding them. This was principally the same in the specimens treated for 5, 10, and 30 min with BrUTP.The dense chromatin usually resided in the middle of the fibrillar centers and was surrounded by a light-fibrous material. An RNase-gold complex did not show any affinity to the fibrillar centers except their periphery. A combination of immunogold labeling for BrUTP incorporation and for DNA localization indicates that the rDNA transcription takes place in the circumferential zone around the dense chromatin. Immunogold labeling of BrUTP incorporation combined with osmium ammine-B staining for DNA suggests that the region around the dense chromatin containing a low concentration of DNA overlapped the site of rDNA transcription. The present data suggest that the transversal structural organization of the nucleolonema consists of the concentric arrangement of at least four different domains: the dense chromatin, the light-fibrous material intermingled with decondensed chromatin, rDNA transcription zone, and putative processing zone of rRNA transcripts.