RNA polymerase II in Cajal bodies of amphibian oocytes

Cajal bodies (coiled bodies) are nuclear organelles that contain a variety of components required for transcription and processing of RNA. Cajal bodies in amphibian oocytes are stained by mAb H14, which recognizes the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II when the heptapeptide repeat is phosphorylated on serine-5. Oocytes were treated with the transcription inhibitor 5, 6-dichloro-1-beta-d-ribofuranosylbenzimidazole (DRB), which prevents phosphorylation of the CTD. Cajal bodies from oocytes that had been treated for 2-3 h with DRB no longer stained with mAb H14, but staining reappeared when the inhibitor was washed out. Epitope-tagged transcripts of two small subunits of polymerase II, RPB6 and RPB9, were injected into the cytoplasm of Xenopus and Triturus oocytes. Newly translated RPB6 and RPB9 were specifically targeted to Cajal bodies within 4 h, and Cajal bodies remained the site of highest concentration of tagged protein during the next 2 days. These data suggest that polymerase subunits pass through the Cajal bodies with a transit time no greater than a few hours. We discuss the possibility that Cajal bodies are sites of assembly or modification of the transcription machinery of the nucleus.