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 chromatinlike 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 B23⌬C, 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 B23⌬C 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.

SummaryCell separation in Bacillus subtilis depends on specific activities of DL-endopeptidases CwlS, LytF and LytE. Immunofluorescence microscopy (IFM) indicated that the localization of LytF depended on its N-terminal LysM domain. In addition, we revealed that the LysM domain efficiently binds to peptidoglycan (PG) prepared by chemically removing wall teichoic acids (WTAs) from the B. subtilis cell wall. Moreover, increasing amounts of the LysM domain bound to TagB-or TagO-depleted cell walls. These results strongly suggested that the LysM domain specifically binds to PG, and that the binding may be prevented by WTAs. IFM with TagB-, TagF-or TagO-reduced cells indicated that LytF-6xFLAG was observed not only at cell separation site and poles but also as a helical pattern along the sidewall. Moreover, we found that LytF was localizable on the whole cell surface in TagB-, TagF-or TagO-depleted cells. These results strongly suggest that WTAs inhibit the sidewall localization of LytF. Furthermore, the helical LytF localization was observed on the lateral cell surface in MreB-depleted cells, suggesting that cell wall modification by WTAs along the sidewall might be governed by an actin-like cytoskeleton homologue, MreB.

Sperm chromatin remodeling after oocyte entry is the essential step that initiates embryogenesis. This reaction involves the removal of sperm-specific basic proteins and chromatin assembly with histones. In mammals, three nucleoplasmin/nucleophosmin (NPM) family proteins–NPM1, NPM2 and NPM3–expressed in oocytes are presumed to cooperatively regulate sperm chromatin remodeling. We characterized the sperm chromatin decondensation and nucleosome assembly activities of three human NPM proteins. NPM1 and NPM2 mediated nucleosome assembly independently of other NPM proteins, whereas the function of NPM3 was largely dependent on formation of a complex with NPM1. Maximal sperm chromatin remodeling activity of NPM2 required the inhibition of its non-specific nucleic acid-binding activity by phosphorylation. Furthermore, the oligomer formation with NPM1 elicited NPM3 nucleosome assembly and sperm chromatin decondensation activity. NPM3 also suppressed the RNA-binding activity of NPM1, which enhanced the nucleoplasm–nucleolus shuttling of NPM1 in somatic cell nuclei. Our results proposed a novel mechanism whereby three NPM proteins cooperatively regulate chromatin disassembly and assembly in the early embryo and in somatic cells.