Cell cycle-dependent association of PML bodies with sites of active transcription in nuclei of mammalian cells.

Promyelocytic leukemia (PML) nuclear bodies constitute one class of intranuclear domains that may be directly involved in the expression of specific genes. Here we have analyzed the spatial relationship between PML bodies and sites of transcriptional activity by indirect immunofluorescence and confocal microscopy during the cell cycle. In unsynchronized mammalian cells approx 30% of PML bodies are spatially associated with transcription sites. These sites contain hyperphosphorylated RNA polymerase II, indicating active mRNA transcription tightly associated with the PML body. In G1 phase of the cell cycle more than 70% of PML bodies contain active transcription foci. A similarly high degree of colocalization (approx 80%) between PML bodies and sites of active transcription was also observed when the cells were exposed to interferon-gamma. We also show that the hypophosphorylated form of RNA polymerase II and the transcriptional coactivator CBP colocalize within PML bodies predominantly in G1. Our observations suggest that PML bodies may be recruited to nuclear sites of induced or up-regulated mRNA transcription where it may serve as a scaffold for factors involved in expression of specific genes.