Synthesis of ribosomes in Saccharomyces cerevisiae.

Abstract

ing of ribosomal precursor RNA by endonuICleases, exonucleases, methylases, pseudou,ridylase, etc.; (v) transcription of genes for 75 or more ribosomatl proteins by RNA polymerase (vi) processing of these transcripts by capping. splicing, and polyadenylation and SuIbsequent migration of the transcripts to the cytoplasm: (vii) translation of' the messenger RNAs (mRNAs) for the ribosomal proteins; (ix) transport of the newly formed ribosomal proteins to the nucleuIS: (x) assembly of these ribosomal proteins with the ri'bosomal precursor RNA and participcation of' the proteins in its processing; and (xi) export of the completed ribosomal subunit to the cytoplasm. To carry out this complex process efficiently, there must be coordination of' synthesis at three levels: among the ri'bosomal proteins, of the ribosomal proteins writh the transcription of rRNA, cand of' RNA and proteins with the biosynthetic needs of the cell. With imagination and ingenuIlty, the Stuidy of ribosome synthesis can lead to an understanding of many of the fund.amental questions about the synthesis, regulation, and movement of macromoleCuIleS within a cell. The purpose of this review is to outline ways in which SuIch fundamnenta,l I problems have been approached a,nd to point out additionatl arecas thcat COuIld be ripe forexploitation. It represents an Updating of a previous review (183), to which the readeris referred f'or a summary of earlier studies: preparation of this review was facilitated by a number of recent reviews of the subject (38, 92, 130-1321, 188, 184). regret that space does not permilt incILuSion of' the synthesis of mitochondrial ribosomes, with their own unique featuires. Nor is there space to deal, except in passing, with the f'unction of the yeacst ri-bosome. The reader is referred in particular to reference 160 for a discussion of suppression by ri'bosome s. rRNA Structure of the rRNA Genes TIhe rRNA genes of the yeast Sacchaoromnvces cerev'isiae are organized in a single tandem array of' 9.1-kilobcase (kb) repeating units on chromosome XII (127). G. Carle and M. V. Olson (personail communication) have recently shown by size determination on pulsed-field gels that in certain strains there arec approximately 2-20 copies of the ribosom'al 256

An S18 ribosomal protein gene copy at the Arabidopsis PFL locus affects plant development by its specific expression in meristems.

“…In humans, it has been postulated that Turner syndrome, a genetic disease typified by short stature and some anatomical anomalies in females, could be caused by haplo-insufficiency of the ribosomal protein S4 that is encoded by an active allele on the X, and one on the Y chromosome (Fisher et al, 1990). In yeast, a decrease in growth rate results from the inactivation of one of the functional gene copies of RP51 or some other ribosomal protein gene pairs (Warner, 1989). The phenotypic alterations seen in the Pfl mutant thus resemble the defects in development upon inactivation of some ribosomal protein genes in other organisms.…”

“…In other organisms, some ribosomal proteins are also encoded by more than one active gene copy. Examples are duplicate genes for several ribosomal proteins in yeast, S4 in humans, and the homolog of yeast S16 in Ascaris lumbricoides (Warner, 1989;Fisher et al, 1990;Etter et al, 1991). In plants, the maize S14, Arabidopsis thaliana L3 and Brassica napus S15 proteins are encoded by small gene families (Larkin et al, 1989;Kim et al, 1990;Bonham-Smith et al, 1992).…”

Depletion of U3 small nucleolar RNA inhibits cleavage in the 5′ external transcribed spacer of yeast pre-ribosomal RNA and impairs formation of 18S ribosomal RNA.

“…Ribosome synthesis is initiated in the nucleolus where RNA polymerase I transcribes a single precursor rRNA (pre-rRNA) containing the domains destined to become the mature 17 -18S, 5.8S and 25-28S rRNAs separated by internal transcribed spacers (ITS1 and ITS2) and flanked by 5' and 3' external transcribed spacers (ETSs). Rapid endonucleolytic cleavages of the pre-rRNA, which in mammals include an initial cleavage within the 5' ETS, result in separation of the small and large subunit precursor components, which are subsequently matured as separate ribonucleoprotein (RNP) complexes (reviewed by Hadjiolov, 1985;Sommerville, 1986;Gerbi et al, 1990;Reeder, 1990; and for yeast by Planta and Raue, 1988;Warner, 1989). Control of the early processing of pre-rRNA is possibly a key stage at which regulation of ribosome synthesis takes place.…”

Heat shock disassembles the nucleolus and inhibits nuclear protein import and poly(A)+ RNA export.

“…The effects of heat shock have frequently been considered a model for regulation of gene expression. In addition to radically shifting the RNA polymerase II transcriptional program in animal cells, heat shock inhibits RNA polymerase I transcription and profoundly alters the structure and function of the nucleolus, leading to a reduction in its size and granularity (Lewis and Pelham, 1985; Warner, 1989;Welsh, 1992;Jordan and Shaw, 1995;Schneiter et al, 1995). Although the reason for the sensitivity of the nucleolus is not known, the nucleolus is a site of intense macromolecular flux.…”

“…Although the reason for the sensitivity of the nucleolus is not known, the nucleolus is a site of intense macromolecular flux. It receives newly synthesized ribosomal proteins and assembles them with rRNA processing intermediates to generate immature ribosomal subunits, which are exported to the cytosol, possibly in association with cyclically shuttling nucleolar proteins (Borer et al, 1989;Warner, 1989;Woolford, 1991;Meier and Blobel, 1992). Since interruption of such macromolecular traffic might underlie perturbation of the nucleolus and since Hsp7O has been implicated in import of proteins into the nucleus of animal cells (Jeoung et al, 1991;Imamoto et al, 1992;Shi and Thomas, 1992;Yang and DeFranco, 1994), we have studied the distribution of 'resident' nucleolar proteins upon heat shock of Saccharomyces cerevisiae.…”