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...