Growth Factors in the Nucleolus?

In 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...