A role for Cajal bodies in assembly of the nuclear transcription machinery.

Cajal bodies (CBs) are small nuclear organelles that contain the three eukaryotic RNA polymerases and a variety of factors involved in transcription and processing of all types of RNA. A number of these factors, as well as subunits of polymerase (pol) II itself, are rapidly and specifically targeted to CBs when injected into the cell. It is suggested that pol I, pol II, and pol III transcription and processing complexes are preassembled in the CBs before transport to the sites of transcription on the chromosomes and in the nucleoli.