A Drosophila anti-RNA polymerase II antibody recognizes a plant nucleolar antigen, RNA polymerase I, which is mostly localized in fibrillar centres.

The distribution of nucleolar RNA polymerase in the nucleolus of onion root meristematic cells has been studied by means of an antibody originally raised against Drosophila RNA polymerase II. This antibody recognizes the homologous domains of the large subunit of the enzyme, which are highly conserved throughout evolution in the three classes of eucaryotic RNA polymerases. Given that RNA polymerase I is confined to the nucleolus, and that the onion cell nucleolus lacks digitations of extranucleolar chromatin, we conclude that the nucleolar enzyme localized is RNA polymerase I. A quantitative approach, independent of the existence of borderlines between nucleolar fibrillar centres and the dense fibrillar component, allowed us to show that the enzyme is localized in fibrillar centres and in the transition area between them and the dense fibrillar component, in parallel with the nucleolar DNA. These results, together with previous autoradiographic, cytochemical and immunocytochemical results, in this and other species, lead us to conclude that the activation of rDNA for transcription occurs in the fibrillar centres and pre-rRNA synthesis is expressed at the transition area between fibrillar centres and the dense fibrillar component. Fibrillar centres are connected to each other by extended RNA polymerase-bound DNA fibres, presumably active in transcription. This work provides evidence of the high evolutionary conservation of some domains of the large subunit of RNA polymerases and of the existence of fibrillar centres in the nucleolus of plant cells, totally homologous to those described in mammalian cells.