Growth Factors in the Nucleolus?

I n eukaryotic cells containing tandem repeated ribosomal RNA genes, there appears a specialized region of chromatin, carrying out gene transcription, rRNA processing, and nascent ribosomal subunit assembly—the nucleolus. One of the first intracellular structures to be described (Montgomery, 1898; Franke, 1988), the nucleolus was established in the 1960s as the center of ribosome synthesis (Schultz, 1966), culminating in the first visualization of genes in action (Miller and Beatty, 1969). Recently it has become evident, however, that the nucleolus is also the site of several nonribosomal RNA processing and assembly functions. These include maturation of signal recognition particle RNA (Jacobson and Pederson, 1998), transfer RNA, U6 small nuclear RNA, telomerase RNA, and some mRNAs (Pederson, 1998). In the envisioned proto-eukaryotic ancestor, various RNA processing and ribonucleoprotein assembly events may have all initially been centered around a minimal genome to chemically facilitate the production of essential gene readout machines. The nucleolus of today’s eukaryotes may be a descendent of this concentrated region of genome readout. Beyond these recently discovered, diverse nonribosomal RNA processing and ribonucleoprotein assembly events in the nucleolus, there is another set of provocative findings: the presence in nucleoli of mitogenic growth factors as well as cell cycle and growth factor-related proteins.