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Summary Changes in the expression and cellular localization of abundant nucleolar proteins: C23/nucleolin, upstream binding factor (UBF), B23/nucleophosmin, and fibrillarin were examined in human lymphocytes subjected to the control of nucleolar activity by phytohemagglutinin and actinomycin D. Data suggest that the up-regulation of ribosomal RNA transcription induced by phytohemagglutinin was accompanied by a significant increase in the nucleolar content of C23/nucleolin, UBF, B23/nucleophosmin, whilst the nucleolar content of fibrillarin had a relatively low-variable. An unraveling of the multicopy ribosomal gene accompanying the mitogenic stimulation was detected through the immunofluorescence of UBF permanently associated with rDNA. Down-regulation of RNA polymerase I activity induced by actinomycin D, 24 hrs after initiating stimulation, did not influence the expression of C23/nucleolin, UBF, and fibrillarin, and up-regulated the expression of B23/nucleophosmin. This inhibition resulted in the translocation of chaperons C23/nucleolin and B23/nucleophosmin to the nucleoplasm, while UBF and fibrillarin persisted in the nucleolus. The reclustering of dispersed transcription units of rDNA, induced by actinomycin D, despite the persistence of UBF in the nucleolus, contradicts the hypothesis that neo-synthesis of UBF is the main drive for unraveling a multicopy rDNA gene. The translocations of C23/nucleolin and B23/nucleophosmin are discussed in relation to the cellular stress response caused by the genotoxic activity of actinomycin D. It is suggested that the reannealing activity of nucleolin and nucleophosmin helps to drive the genotoxic agent to the nucleolus, diminishes the diversity of genotoxic damage and inhibits the growth-division activity of cells.

Expression and translocation of major nucleolar proteins in relation to the transcriptional activity of the nucleolus