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12 Control of rRNA and tRNA Production Is Closely Tied to Cell Growth

Robert J. White


In actively growing cells, up to 80% of nuclear transcription can be taken up by the synthesis of rRNA and tRNA (Moss and Stefanovsky 2002). Indeed, these stable transcripts constitute ~95% of a cell’s RNA content (Warner 1999). The Herculean task of supplying large rRNA is performed in eukaryotes by RNA polymerase (Pol) I, which is dedicated solely to manufacturing a single transcript that is then processed into the 18S, 5.8S, and 28S rRNA products. Pol III is highly specialized for the synthesis of short untranslated transcripts, including tRNA, 5S rRNA, and 7SL RNA (for review, see White 2002). Transcription of the protein-encoding genes is carried out by Pol II, which is smaller than Pols I and III and has fewer subunits (Sentenac 1985).

Whereas the genetic templates for Pols II and III are dispersed throughout the genome, those of Pol I are clustered in highly reiterated tandem arrays (for review, see Paule 1998). These rRNA genes are so intensely transcribed that the resulting dense concentration of macro-molecules can be seen under a light microscope as nucleoli, the most prominent visible features of a nucleus. Nucleoli can be regarded as dynamic ribosome factories, in which rRNA is synthesized in the fibrillar centers and then assembled into ribosomes in the surrounding granular regions (for review, see Carmo-Fonseca et al. 2000). Although Pol III transcription occurs in the nucleoplasm, many of its products, including tRNA, move to nucleoli for processing and maturation (Jacobson et al. 1997; Bertrand et al. 1998; Carmo-Fonseca et...

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