Regulation of nucleolar chromatin by B23/nucleophosmin jointly depends upon its RNA binding activity and transcription factor UBF.
Histone chaperones regulate the density of incorporated histone proteins around DNA transcription sites and therefore constitute an important site-specific regulatory mechanism for the control of gene expression. At present, the targeting mechanism conferring this site specificity is unknown. We previously reported that the histone chaperone B23/nucleophosmin associates with rRNA chromatin (r-chromatin) to stimulate rRNA transcription. Here, we report on the mechanism for site-specific targeting of B23 to the r-chromatin. We observed that, during mitosis, B23 was released from chromatin upon inactivation of its RNA binding activity by cdc2 kinase-mediated phosphorylation. The phosphorylation status of B23 was also shown to strongly affect its chromatin binding activity. We further found that r-chromatin binding of B23 was a necessary condition for B23 histone chaperone activity in vivo. In addition, we found that depletion of upstream binding factor (UBF; an rRNA transcription factor) decreased the chromatin binding affinity of B23, which in turn led to an increase in histone density at the r-chromatin. These two major strands of evidence suggest a novel cell cycle-dependent mechanism for the site-specific regulation of histone density via joint RNA- and transcription factor-mediated recruitment of histone chaperones to specific chromosome loci.
Hisaoka
Miharu
M
Graduate School of Comprehensive Human Science, University of Tsukuba, Tsukuba, Japan.
Ueshima
Shuhei
S
Murano
Kensaku
K
Nagata
Kyosuke
K
Okuwaki
Mitsuru
M
D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D016203:CDC2 Protein Kinase; D002453:Cell Cycle; D002460:Cell Line; D002466:Cell Nucleolus / Q000235:genetics / Q000378:metabolism*; D002843:Chromatin / Q000235:genetics / Q000378:metabolism*; D019926:Cyclin B / Q000235:genetics / Q000378:metabolism; D018844:Cyclin-Dependent Kinases; D017931:DNA Primers / Q000235:genetics; D006367:HeLa Cells; D006801:Humans; D008938:Mitosis / Q000235:genetics / Q000502:physiology; D018832:Molecular Chaperones / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D009687:Nuclear Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D000090243:Nucleophosmin; D010766:Phosphorylation; D035463:Pol1 Transcription Initiation Complex Proteins / Q000235:genetics / Q000378:metabolism*; D012313:RNA / Q000235:genetics / Q000378:metabolism*; D012335:RNA, Ribosomal / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic
D002843:Chromatin; D019926:Cyclin B; D017931:DNA Primers; D018832:Molecular Chaperones; D050505:Mutant Proteins; C000716967:NPM1 protein, human; D009687:Nuclear Proteins; D035463:Pol1 Transcription Initiation Complex Proteins; D012335:RNA, Ribosomal; D034741:RNA, Small Interfering; D011994:Recombinant Proteins; C063844:transcription factor UBF; D000090243:Nucleophosmin; D012313:RNA; D016203:CDC2 Protein Kinase; C528899:CDK1 protein, human; D018844:Cyclin-Dependent Kinases