RNA polymerase II condensate formation and association with Cajal and histone locus bodies in living human cells

Genes Cells. 2021 May;26(5):298-312. doi: 10.1111/gtc.12840. Epub 2021 Mar 19.


In eukaryotic nuclei, a number of phase-separated nuclear bodies (NBs) are present. RNA polymerase II (Pol II) is the main player in transcription and forms large condensates in addition to localizing at numerous transcription foci. Cajal bodies (CBs) and histone locus bodies (HLBs) are NBs that are involved in transcriptional and post-transcriptional regulation of small nuclear RNA and histone genes. By live-cell imaging using human HCT116 cells, we here show that Pol II condensates (PCs) nucleated near CBs and HLBs, and the number of PCs increased during S phase concomitantly with the activation period of histone genes. Ternary PC-CB-HLB associates were formed via three pathways: nucleation of PCs and HLBs near CBs, interaction between preformed PC-HLBs with CBs and nucleation of PCs near preformed CB-HLBs. Coilin knockout increased the co-localization rate between PCs and HLBs, whereas the number, nucleation timing and phosphorylation status of PCs remained unchanged. Depletion of PCs did not affect CBs and HLBs. Treatment with 1,6-hexanediol revealed that PCs were more liquid-like than CBs and HLBs. Thus, PCs are dynamic structures often nucleated following the activation of gene clusters associated with other NBs.

Keywords: Cajal body; RNA polymerase II; histone locus body; liquid droplet; live-cell imaging; transcription.

MeSH terms

  • Cell Survival / drug effects
  • Coiled Bodies / drug effects
  • Coiled Bodies / metabolism*
  • Glycols / pharmacology
  • Green Fluorescent Proteins / metabolism
  • HCT116 Cells
  • Histones / metabolism*
  • Humans
  • Models, Biological
  • Nuclear Proteins / metabolism
  • RNA Polymerase II / metabolism*
  • S Phase / drug effects


  • Glycols
  • Histones
  • Nuclear Proteins
  • enhanced green fluorescent protein
  • p80-coilin
  • Green Fluorescent Proteins
  • RNA Polymerase II
  • hexamethylene glycol