Control of cell division in Saccharomyces cerevisiae by methionyl-tRNA.

Abstract

We suggest that two events are necessary for an asynchronous population of cells to undergo arrest in the GI phase of the cell cycle upon nutrient starvation. First, passage through GI must be prevented by a deficiency of some metabolic intermediate. Since this intermediate may act indirectly to arrest division, we designate it the "signal." We have found three conditions under which Saccharomyces cerevisiae cells arrest division in GI: sulfate starvation of a prototroph, methionine starvation of an auxotroph, or a shift of a conditional methionyl-tRNA synthetase mutant [ILmethionine:tRNAMet ligase (AMP-forming), EC 6.1.1.10 Ito a restrictive condition. We interpret these results to indicate that the signal for sulfate starvation in S. cerevisiae is generated near the end of the sulfate assimilation pathway (at or beyond the formation of methionyl-tRNA). As a unifying hypothesis, we propose that the signal for all nutrients is generated at the level of protein biosynthesis.A second event necessary for GI arrest is the provision of sufficient protein synthetic capacity for cells to finish the cycles that are in progress when the signal is generated. This necessity is demonstrated by the failure of the methionyl-tRNA synthetase mutant to undergo GI arrest when protein synthesis is abruptly terminated by a shift to 36°into methionine-deficient medium.Many eukaryotic cells pass reversibly from a proliferative to a non-proliferative state and, in most cases that have been studied, division is arrested in the G1 interval of the cell cycle (1-3). Since growth is usually required for division, the cells of many organisms, including prokaryotic (4) and eukaryotic (5-9) microorganisms, metazoa (10-17), and metaphyta (18), control division in response to inorganic and organic nutrients. Some evidence exists to suggest that even the protein growth factors of mammalian cells such as serum factors and agglutinins may act by enhancing the availability of small molecular weight nutrients (19,20 (mesl-). Strains with the prefix DU were constructed by first mating a haploids to the a-haploid EMS-63 and then producing homozygosity of the mutation for methionine auxotrophy by x-ray-induced mitotic recombination (25). The haploid EMS-63 was supplied by Dr. Gerald Fink (Cornell University).The pathway of methionine biosynthesis has been discussed by Masselot and Robichon-Szulmajster (26). The methionine auxotrophs used in this paper are representative of the possible levels in the methionine biosynthetic pathway. Only three gene-enzyme assignments have been documented; the met2-mutant is defective in homoserine-O-transacetylase, met25-is defective in homocysteine synthetase, and mesa is defective in methionyl-tRNA synthetase [L-methionine:tRNAMet ligase (AMP-forming), EC 6.1. 1.101 (26, 27). Strains carrying the mes mutation exhibit two phenotypes, a methionine auxotrophy and a temperature-sensitivity. We have undertaken a genetic and biochemical study of this mutation. The results show that both phenotypes are due to a sing...

A Survey of Essential Gene Function in the Yeast Cell Division Cycle

“…The intimate relationship between protein biosynthesis and cell cycle progression through G1 that we observed is consistent with previous observations linking protein synthesis with G1 transit (Unger and Hartwell, 1976;Bedard et al, 1981;Moreno and Nurse, 1994), with connections between ribosome biogenesis and cell size regulation at Start (Jorgensen et al, 2002(Jorgensen et al, , 2004, and with links between translation rate and G1 (Polymenis and Schmidt, 1997) and provides a global view of cellular processes that contribute to passage through the cell cycle commitment point in G1, Start. There is concordance between our G1 data and that derived from cell cycle screening of Drosophila cells after gene product depletion by RNAi (Bjorklund et al, 2006) in that both screens show clear involvement of protein biosynthesis pathways in G1 transit.…”

“…For example, depletion of SSU processome proteins leads to G1 arrest due to lack of ribosomes (Bernstein and Baserga, 2004), as does depletion of the 20S pre-rRNA maturation factor Rio1 (Angermayr et al, 2002). Similarly, tRNA charging has long been linked to G1 transit as a number of mutants in tRNA synthetases arrest in G1 (Unger and Hartwell, 1976;Bedard et al, 1981). Consistent with existing links between protein synthesis and G1 transit, our data indicate that the most dramatic restriction on a cell's ability to progress past the cell cycle commitment point in G1 (known as Start in yeast) is the capacity for protein synthesis, as inhibiting protein synthesis by a range of different means caused accumulation of cells in G1.…”

Ribosome Biogenesis Is Sensed at the Start Cell Cycle Checkpoint

“…Alternatively, yeast could be sensing a balance between new 60S and 40S subunit synthesis or decreases in new 40S subunit synthesis. In addition, we cannot exclude the possibility that yeast sense small changes in translation that are undetectable with our assays because changes in translation can clearly influence Start (Unger and Hartwell, 1976;Moore, 1988).…”

“…Similar phenomena exist in a wide variety of organisms including bacteria, fission yeast, and avian erythroblasts (Donachie, 1968;Hartwell et al, 1974;Johnston et al, 1977;Nurse and Thuriaux, 1977;Dolznig et al, 2004). Arrest at Start in yeast can be caused by nutrient deprivation, mating pheromones, or translation defects (Unger and Hartwell, 1976;Hartwell and Unger, 1977;Johnston et al, 1977). It is not known whether the critical cell size setpoint sensed corresponds to cell volume, protein content per cell, RNA content per cell, or rates of protein or ribosome synthesis (Jorgensen and Tyers, 2004).…”

Comprehensive Identification of Cell Cycle–regulated Genes of the YeastSaccharomyces cerevisiaeby Microarray Hybridization

“…First, the pool of available cellular methionine is smaller than virtually any other amino acid; thus, methionine is likely to be limiting (Jones and Fink, 1982). Indeed, Unger and Hartwell (1976) noted that starvation for sulfur or for methionine effectively causes G1 arrest, suggesting that cell cycle progression is particularly sensitive to the availability of methionine. They also found that a temperaturesensitive allele of methionine tRNA synthetase causes G1 arrest, even in the presence of methionine.…”