Staged assembly of histone gene expression machinery at subnuclear foci in the abbreviated cell cycle of human embryonic stem cells

Human embryonic stem (hES) cells have an abbreviated G₁ phase of the cell cycle. How cells expedite G₁ events that are required for the initiation of S phase has not been resolved. One key regulatory pathway that controls G₁/S-phase transition is the cyclin E/CDK2-dependent activation of the coactivator protein nuclear protein, ataxia-telangiectasia locus/histone nuclear factor-P (p220^NPAT/HiNF-P) complex that induces histone gene transcription. In this study, we use the subnuclear organization of factors controlling histone gene expression to define mechanistic differences in the G₁ phase of hES and somatic cells using in situ immunofluorescence microscopy and fluorescence in situ hybridization (FISH). We show that histone gene expression is supported by the staged assembly and modification of a unique subnuclear structure that coordinates initiation and processing of transcripts originating from histone gene loci. Our results demonstrate that regulatory complexes that mediate transcriptional initiation (e.g., p220^NPAT) and 3′-end processing (e.g., Lsm10, Lsm11, and SLBP) of histone gene transcripts colocalize at histone gene loci in dedicated subnuclear foci (histone locus bodies) that are distinct from Cajal bodies. Although appearance of CDK2-phosphorylated p220^NPAT in these domains occurs at the time of S-phase entry, histone locus bodies are formed ≈1 to 2 h before S phase in embryonic cells but 6 h before S phase in somatic cells. These temporal differences in the formation of histone locus bodies suggest that the G₁ phase of the cell cycle in hES cells is abbreviated in part by contraction of late G₁.