Distribution of B-36 nucleolar protein in relation to transcriptional activity in plant cells

Abstract

The Mr 34000 nucleolar protein B-36 was immunolocalized using a mouse monoclonal antibody in different plant cells by light and electron microscopy. Specific immunofluorescence was observed in nucleoli of cryofixed and cryosectioned material. Immunoelectron microscopy was performed on ultrathin cryosections and Lowicryl sections. Immunogold labelling revealed that B-36 is mainly localized in the nucleolar dense fibrillar component (DFC). The fibrillar centres and the granular component were practically devoid of gold particles. Nucleoli with different rates of transcriptional activity and, as a consequence, with different morphologies showed the same immunolocalization. This was true for both somatic and germinal tissues of the different plant species studied: Allium cepa root meristematic cells and Scilla peruviana and Capsicum annuum pollen grains. The presence of the B-36 antigen in the DFC of nucleoli with very different rates of transcriptional activity indicates that it is a constitutive protein. Moreover, the localization suggests that it could play a role in the transcription and/or early processing of rRNA. The presence of B-36 in the different plant tissues and species studied, confirms once more its high level of conservation throughout evolution. Another Mr 34000 protein called fibrillarin was also found in the DFC. Taking these and other published data together, we think that the antibodies used in the different studies are probably binding different epitopes of identical or very similar Mr 34000 nucleolar proteins.