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Ribosomal proteins are ubiquitous, abundant, and RNA-binding, prime candidates for recruitment to extra-ribosomal functions. Indeed, they participate in balancing the synthesis of the RNA and protein components of the ribosome itself. An exciting new story is that ribosomal proteins are sentinels for the self-evaluation of cellular health. Perturbation of ribosome synthesis frees ribosomal proteins to interface with the p53 system, leading to cell cycle arrest or to apoptosis. Yet in only a few cases can we clearly identify the recruitment of ribosomal proteins for other extra-ribosomal functions. Is this due to a lack of imaginative evolution by cells and viruses, or to a lack of imaginative experiments by molecular biologists?

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The 138 genes encoding the 79 ribosomal proteins (RPs) of Saccharomyces cerevisiae form the tightest cluster of coordinately regulated genes in nearly all transcriptome experiments. The basis for this observation remains unknown. We now provide evidence that two factors, Fhl1p and Ifh1p, are key players in the transcription of RP genes. Both are found at transcribing RP genes in vivo. Ifh1p, but not Fhl1p, leaves the RP genes when transcription is repressed. The occupancy of the RP genes by Ifh1p depends on its interaction with the phospho-peptide recognizing forkhead-associated domain of Fhl1p. Disruption of this interaction is severely deleterious to ribosome synthesis and cell growth. Loss of functional Fhl1p leads to cells that have only 20% the normal amount of RNA and that synthesize ribosomes at only 5-10% the normal rate. Homeostatic mechanisms within the cell respond by reducing the transcription of rRNA to match the output of RPs, and by reducing the global transcription of mRNA to match the capacity of the translational apparatus.

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To explore the regulatory elements that maintain the balanced synthesis of the components of the ribosome, we isolated a temperature-sensitive (ts) mutant of Saccharomyces cerevisiae in which transcription both of rRNA and of ribosomal protein genes is defective at the nonpermissive temperature. Temperature sensitivity for growth is recessive and segregates 2:2. A gene that complements the ts phenotype was cloned from a genomic DNA library. Sequence analysis revealed that this gene is SLY), encoding a protein essential for protein and vesicle transport between the endoplasmic reticulum and the Golgi apparatus. In the strain carrying our ts allele of SLYI, accumulation of the carboxypeptidase Y precursor was detected at the nonpermissive temperature, indicating that the secretory pathway is defective. To ask whether the eflect of the ts allele on ribosome synthesis was specific for sly) or was a general result of the inactivation of the secretion pathway, we assayed the levels of mRNA for several ribosomal proteins in cells carrying ts alleles of secl, sec7, secil, secl4, secl8, sec53, or sec63, representing all stages of secretion. In each case, the mRNA levels were severely depressed, suggesting that this is a common feature in mutants of protein secretion. For the mutants tested, transcription of rRNA was also substantially reduced. Furthermore, treatment of a sensitive strain with brefeldin A at a concentration sufficient to block the secretion pathway also led to a decrease of the level of ribosomal protein mRNA, with kinetics suggesting that the effect of a secretion defect is manifest within 15 to 30 min. We conclude that the continued function of the entire secretion pathway is essential for the maintenance of ribosome synthesis. The apparent coupling of membrane synthesis and ribosome synthesis suggests the existence of a regulatory network that connects the production of the various structural elements of the cell.

We investigated the regulation of ribosome synthesis in Saccharomyces cerevisiae growing at different rates and in response to a growth stimulus. The ribosome content and the rates of synthesis of ribosomal ribonucleic acid and of ribosomal proteins were compared in cultures growing in minimal medium with either glucose or ethanol as a carbon source. The results demonstrated that ribosome content is proportional to growth rate. Moreover, these steady-state concentrations are regulated at the level of synthesis of ribosomal precursor ribonucleic acid and of ribosomal proteins. When cultures growing on ethanol were enriched with glucose, the rate of ribosomal ribonucleic acid synthesis, measured by pulsing cells with [methyl-3H]methionine, increased by 40% within 5 min, doubled within 15 min, and reached a steady state characteristic of the new growth medium by 30 min. Labeling with [3H]leucine revealed a coordinate increase in the rate of synthesis of 30 or more ribosomal proteins as compared with that of total cellular proteins. Their synthesis was stimulated approximately 2.5-fold within 15 min and nearly 4-fold within 60 min. The data suggest that S. cerevisiae responds to a growth stimulus by preferential stimulation of the synthesis of ribosomal ribonucleic acid and ribosomal proteins.The complex nature of the ribosome and its central role in cell growth and maintenance have generated great interest in the regulation of its synthesis. In Escherichia coli, many aspects of the physiology of bacterial growth were established by studying the synthesis of ribosomes in cultures growing exponentially on different media and after transition from one medium to another (10,12,14). In particular, it was shown that the concentration of ribosomes in cells is proportional to the rates of growth and of protein synthesis (9). Other studies demonstrated that the rate of total protein synthesis per unit of ribosomal ribonucleic acid (RNA) is constant over a wide range of growth conditions, suggesting that the rate of growth in bacteria is limited by the concentration of ribosomes in the cell (5,12,16). Moreover, when cells are shifted from a nutritionally poor medium to a rich medium (shift-up), they exhibit an instantaneous acceleration of synthesis of both ribosomal RNA and ribosomal proteins, leading to a rapid accumulation of ribosomes until the concentration characteristic of the new growth rate is established (3,12,15,18). Although the mechanisms responsible for the coordinate response of ribosomal RNA and ribosomal protein synthesis to such a shift are not yet known, the phenomenon has provided a system for examining the strategies employed by the cell to modulate its production of ribosomes and has led to substantial insight into the relationship between ribosomes and growth control.We wished to examine a similar situation in the eucaryotic cell Saccharomyces cerevisiae and to explore the generality of the phenomenon of modulation of ribosome concentration with growth rate. Published reports of growth ratedependent ch...

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