The nucleolus and the four ribonucleoproteins of translation

The classical view of the nucleolus as solely committed to ribosome biosynt hesis 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 cel lular senescence. The signal recognition particle (SRP)(1) RNA, and several protein components of the SRP also recently have been detected in the nucl eolus. Thus, the large and small ribosomal subunits, the 5S rRNA-ribonucleo protein complex, and now the SRP, are known to be assembled in or pass thro ugh the nucleolus. These findings, together with the recent observations th at some transfer RNA precursor molecules and the pretransfer RNA processing enzyme, RNase P, are also found in the nucleolus, raise the possibility th at these translational components are congressed in the nucleolus in order to probatively interact with one another, perhaps as a test of proper confo rmational fit. We hypothesize that such interactions may be an important ch eckpoint 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.