<i>met17</i>Δ yeast show density-dependent growth on sulfate.

(A) Schematic summarizing sulfate assimilation in S. cerevisiae. Sulfate SO42− is reduced to sulfide S2− (orange) through a series of enzymatic reactions, and Met17 then combines sulfide with a nitrogenous compound (O-acetyl homoserine) to generate homocysteine. All organosulfurs (purple) are interconvertible via the transsulfuration pathway (cysteine, glutathione) and the methyl cycle (methionine, AdoMet). Solid and dotted lines represent one-step and multistep reactions, respectively. Note that according to this conventional understanding, met17 mutants should not be able to generate organosulfurs from sulfate and, therefore, should require supplementation of organosulfur in their growth media. (B) Auxotrophy of met17Δ is leaky in both S288C and RM11 backgrounds. On agar plates containing synthetic minimal medium (SD: with sulfate, but without organosulfurs), patches of prototrophic yeast grew a dense lawn and patches of the uracil auxotroph ura3Δ remained clear, while patches of met17Δ showed papillae or patchy growth. Plate was imaged 3 days after patching. Prototrophic strains were WY2601 and WY1870, met17Δ were WY2590 and WY2548, and ura3Δ were WY572 and WY1318. (C) met17Δ yeast can be repeatedly passaged on sulfate. A liquid culture of met17Δ (WY2035) in SD medium could persistently grow to saturation upon repeated dilutions (arrowheads). (D) met17Δ show density-dependent growth on sulfate. Darker shades of gray represent higher initial cell densities. At each starting density, 3 identical liquid cultures were set up. For met17Δ (middle panel; WY2035), all 3 cultures at higher initial cell densities grew deterministically, while stochasticity in lag time was observed at intermediate cell densities (green asterisks) and the lowest cell densities did not grow. In contrast, prototrophs (left panel; WY1870) grew to saturation from all initial densities, while ura3Δ (right panel; WY1318) showed only residual growth at any density when uracil was withheld. In (C) and (D), strains were of RM11 background and population growth was recorded as optical density at 600 nm (OD600). Each trendline represents a 7-ml culture in an 18-mm diameter glass tube with a loosely fitted plastic cap. WY2035 require lysine supplementation in SD medium, which was maintained throughout. The data underlying this figure can be found in S1 Data.