THE PLURIFUNCTIONAL NUCLEOLUS

The nucleolus of eukaryotic cells was first described in the early 19t h century and was discovered in the 1960s to be the seat of ribosome s ynthesis. Although rRNA transcription, rRNA processing and ribosome as sembly have been clearly established as major functions of the nucleol us, recent studies suggest that the nucleolus participates in many oth er aspects of gene expression as well. Thus, the nucleolus has been im plicated in the processing or nuclear export of certain mRNAs, In addi tion, new results indicate that biosyntheses of signal recognition par ticle RNA and telomerase RNA involve a nucleolar stage and that the nu cleolus is also involved in processing of U6 RNA, one of the spliceoso mal small nuclear RNAs, Interestingly, these three nucleolus-associate d small nuclear RNAs (signal recognition particle RNA, telomerase RNA and U6 RNA) are components of catalytic ribonucleoprotein machines. Fi nally, recent work has also suggested that some transfer RNA precursor s are processed in the nucleolus. The nucleolus may have evolutionaril y descended from a proto-eukaryotic minimal genome that was spatially linked to vicina! RNA processing and ribonucleoprotein assembly events involved in gene read-out. The nucleolus of today's eukaryotes, now s urrounded by the chromatin of over 2 billion years of genome expansion , may still perform these ancient functions, in addition to ribosome b iosynthesis, The plurifunctional nucleolus concept has a strong footin g in contemporary data and adds a new perspective to our current pictu re of the spatial-functional design of the cell nucleus.