Single ribosomal transcription units are linear, compacted Christmas treesin plant nucleoli

Citation
P. Gonzalez-melendi et al., Single ribosomal transcription units are linear, compacted Christmas treesin plant nucleoli, PLANT J, 27(3), 2001, pp. 223-233
Citations number
36
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Plant Sciences","Animal & Plant Sciences
Journal title
PLANT JOURNAL
ISSN journal
0960-7412 → ACNP
Volume
27
Issue
3
Year of publication
2001
Pages
223 - 233
Database
ISI
SICI code
0960-7412(200108)27:3<223:SRTUAL>2.0.ZU;2-F
Abstract
The rDNA transcription units are enormous macromolecular structures located in the nucleolus and containing 50-100 RNA polymerases together with the n ascent pre-rRNA attached to the rDNA. It has not previously been possible t o visualize nucleolar transcription units directly in intact nucleoli, alth ough highly spread preparations in the electron microscope have been imaged as 'Christmas trees' 2-3 mum long. Here we determine the relative conforma tion of individual transcription units in Pisum sativum plant nucleoli usin g a novel labelling technique. Nascent transcripts were detected by a highl y sensitive silver-enhanced 1 nm gold procedure, followed by 3D electron mi croscopy of entire nucleoli. Individual transcription units are seen as con ical, elongated clusters approximately 300 nm in length and 130 nm in width at the thickest end. We further show that there were approximately 300 act ive ribosomal genes in the nucleoli examined. The underlying chromatin stru cture of the transcribing rDNA was directly visualized by applying a novel limited extraction procedure to fixed specimens in order to wash out the pr oteins and RNA, thus specifically revealing DNA strands after uranyl acetat e staining. Using this technique, followed by post-embedding in situ hybrid ization, we observed that the nucleolar rDNA fibres are not extended but sh ow a coiled, thread-like appearance. Our results show for the first time th at native rDNA transcription units are linear, compacted Christmas trees.