The transcription unit of ribosomal genes is attached to the nuclear skeleton

The relationship between various loci of the ribosomal gene repeat and the nucleoskeleton was examined in agarose-embedded HeLa cells. The accessibility of intranucleolar structures to molecular probes was improved by dispersing the granular component of nucleoli, and unattached DNA was removed from permeabilized nuclei under "physiological" conditions by enzymatic digestion and subsequent electroelution. The cells were then hybridized in situ with various human rDNA probes for the transcription unit or for the intergenic spacer. A strong signal was detected with probes for the transcription unit but no signal was seen with probes for the intergenic spacer. These results show that only the transcription unit is strongly attached to the nucle(ol)ar skeleton and imply that rDNA is probably attached to the skeleton primarily via RNA polymerase complexes rather than via sequence-specific attachment sites. Nucleolar fibrillar centers, embedded into the nucle(ol)ar skeleton, provide structural support for these attachments.