Joseph G. Gall

The Cajal Body: Assembly Site for the Nuclear Transcription Machinery

The first step in the expression of any gene is the formation of an RNA copy of its DNA. This step, referred to as transcription, takes place in the cell nucleus. Transcription requires the enzyme RNA polymerase and a multitude of processing factors, which modify the primary transcript before it is exported to the cytoplasm as the functional messenger RNA. Although the biochemical details of transcription and processing of RNA are known in considerable detail, relatively little is known about their cellular organization. It has been generally assumed that the polymerase and various additional factors are recruited separately to active genes on the chromosomes. Researchers in the Gall lab have recently proposed an alternative model in which the RNA machinery is preassembled as a unitary particle completely separate from the chromosomes. This particle, which they call a transcriptosome, is recruited to the sites of active genes, bringing the entire transcription machinery to the genes as a single packet. This model is based on cytological and molecular studies of animal cell nuclei.

Much of the lab's work is carried out on oocytes of the frog Xenopus. An oocyte is an egg before it is laid by the female; in Xenopus it is a single giant cell up to 1.5 mm in diameter. The nucleus of the oocyte is equally large, about 0.4 mm in diameter, and can be hand-isolated from the oocyte. For historical reasons, the oocyte nucleus is usually called the germinal vesicle, or simply GV. The large size of the GV permits one to examine the contents of a cell nucleus in unprecedented morphological detail. At the same time, many biochemical studies can be carried out on single hand-isolated GVs or small numbers of GVs.

The scientists have focused their attention on structures in the GV called Cajal bodies, first seen in the nuclei of nerve cells nearly 100 years ago by the Spanish neurobiologist Ram—n y Cajal. They have shown that Cajal bodies contain many, if not all, factors required for transcription and processing of messenger RNA, including RNA polymerase II and factors required for initiation, splicing, cleavage, and polyadenylation of the RNA transcript. Remarkably, Cajal bodies also contain the special factors required for transcription and processing of ribosomal RNA (polymerase I and its associated factors), as well as polymerase III, which transcribes certain small RNAs. The GV is an ideal system in which to study not only the sites where specific factors are found, but also where they are assembled into macromolecular complexes. This can be done by injecting factors into the cell and observing where they go initially, and where they end up after a longer time. From experiments of this sort, the scientists conclude not only that the transcription machinery exists in the Cajal bodies, but that a significant fraction of it must be assembled there.

In future experiments, the Gall lab hopes to learn how various factors are recruited to the Cajal bodies and how they assemble into the large transcriptosome complexes. The researchers are also trying to isolate transcriptosomes from oocytes to determine their composition and structure in more detail.