SITES OF RDNA TRANSCRIPTION ARE WIDELY DISPERSED THROUGH THE NUCLEOLUS IN PISUM-SATIVUM AND CAN COMPRISE SINGLE GENES

Incorporation by RNA polymerases of BrUTP into both plant root tissue and isolated plant nuclei as a method for localization of the sites of transcription has been used. In this paper pea root tissue was used, and under the conditions employed, nearly all the incorporation occurs in the nucleolus, and thus must be catalysed by RNA polymerase I. Imm unofluorescence and confocal microscopy shows that incorporation occur s in a pattern consisting of many small foci distributed widely throug h the dense fibrillar component of the nucleoli. Immunogold labelling using silver-enhanced Nanogold probe at the electron microscopic level confirms the sites of transcription as small foci approximately 200nm in diameter. Simultaneous fluorescence in situ hybridization with a p robe to the external transcribed spacer (ETS) region of the pre-rRNA s hows that the structures revealed by this probe and the BrUTP immunofl uorescence labelling are Very similar. A probe to the transcribed port ion of the rDNA (18S) also shows a good correlation to the sites of Br UTP incorporation within the nucleolus. On the other hand a probe to t he non-transcribed intergenic spacer region (NTS) shows very little co incidence with the sites of BrUTP incorporation, and double fluorescen ce in situ labelling with both 18S and NTS probes confirms this differ ence in localization. These results suggest that most BrUTP foci corre spond to single transcribed genes.