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 a ccessibility of intranucleolar structures to molecular probes was impr oved by dispersing the granular component of nucleoli, and unattached DNA was removed from permeabilized nuclei under ''physiological'' cond itions by enzymatic digestion and subsequent electroelution. The cells were then hybridized in situ with various human rDNA probes for the t ranscription unit or for the intergenic spacer. A strong signal was de tected with probes for the transcription unit but no signal was seen w ith probes for the intergenic spacer. These results show that only the transcription unit is strongly attached to the nucle(ol)ar skeleton a nd 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 skel eton, provide structural support for these attachments. (C) 1996 Acade mic Press, Inc.