THE NUCLEOLUS - AN ORGANELLE FORMED BY THE ACT OF BUILDING A RIBOSOME

Recent evidence corroborates the idea that the structure of the nucleo lus need not be strictly maintained for proper function, suggesting th at the organelle is composed of supramolecular assemblies formed durin g rRNA synthesis. More controversial is whether the nucleolus exists i n the absence of rRNA synthesis and whether it interacts with the nucl ear scaffold. The simultaneous and highly integrative nature of buildi ng a ribosome is reflected in the numerous observations showing that p roteins involved in all aspects of ribosomal biogenesis affect pre-rRN A processing. The identification of several new nucleolar proteins wit hout an obvious role in pre-rRNA metabolism may provide the field with long sought after assembly factors that might be key players in eukar yotic ribosome biogenesis.