It is well established that the transcription rate of the rRNA gene is closely associated with profound alterations in the cell growth rate. Regulation of rRNA gene transcription is likely to be dependent on the dynamic conversion of the chromatin structure. Previously, we identified B23/nucleophosmin, a multifunctional nucleolar phosphoprotein, as a component of template activating factor III that remodels the chromatin-like structure of the adenovirus genome complexed with viral basic proteins. It has also been shown that B23 has histone chaperone activity. Here, we examined the effect of B23 on rRNA gene transcription. B23 was found to be associated with the rRNA gene chromatin. Small-interfering-RNA-mediated down-regulation of the B23 expression level resulted in reduction of the transcription rate of the rRNA gene. We constructed a B23 mutant termed B23ΔC, which lacks the domain essential for the histone chaperone activity and inhibited the histone binding activity of B23 in a dominant-negative manner. Expression of B23ΔC decreased rRNA gene transcription and the rate of cell proliferation. These results suggest that B23 is involved in the transcription regulation of the rRNA gene as a nucleolar histone chaperone.
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