The nucleolus and the four ribonucleoproteins of translation

The classical view of the nucleolus as solely committed to ribosome biosynthesis has been modified by recent studies pointing to additional roles for this nuclear domain. These newly recognized features include the nucleolar presence of several nonribosomal RNAs transcribed by RNA polymerase III, as well as nucleolar roles in gene silencing, cell cycle progression, and cellular senescence. The signal recognition particle (SRP)(1) RNA, and several protein components of the SRP also recently have been detected in the nucleolus. Thus, the large and small ribosomal subunits, the 5S rRNA-ribonucleoprotein complex, and now the SRP, are known to be assembled in or pass through the nucleolus. These findings, together with the recent observations that some transfer RNA precursor molecules and the pretransfer RNA processing enzyme, RNase P, are also found in the nucleolus, raise the possibility that these translational components are congressed in the nucleolus in order to probatively interact with one another, perhaps as a test of proper conformational fit. We hypothesize that such interactions may be an important checkpoint during nucleolar assembly of the translational machinery at steps ranging from the regulation of nascent transcript processing to a possible transient preassembly of the entire translational apparatus.