Figure 3. Glycerol uptake involves derepression of glycerol catabolic genes.

a. Known regulation of glycerol uptake under aerobic conditions. Blue ovals represent enzymes, green rectangles transcriptional regulators. Dashed lines indicate allosteric inhibition^18,19,49. See Supplementary Figure 4 for more details.

b. LacZ reporter expression from the glpFK promoter versus growth rate in titratable PtsG strains (HE305 and mutants thereof) with varying 3MBA levels. In the presence of glucose + glycerol (purple squares) expression fell abruptly if the growth rate exceeded that on glycerol alone (orange square). In contrast, in the ΔglpR or ΔglpR glpK22 background (pale blue diamonds and green triangles) the expression followed a straight line (a “C-line”¹⁶). (Regression lines are shown. ΔglpR strain:$R_{\text{adj}}^2=0.98$, n = 6 conditions, t = 17.6, df = 4, two-sided p = 6.1 × 10⁻⁵. ΔglpR glpK22 strain:$R_{\text{adj}}^2=0.99$, n = 6 conditions, t = 46.0, df = 4, two − sided p = 1.3 × 10⁻⁶.) Little expression was observed on glucose alone (blue squares).

c. As Panel b, except that expression from the glpD promoter was measured (strain HE397 and mutants thereof). Again, in the presence of glucose + glycerol (purple squares) expression turned off abruptly if the growth rate exceeded that on glycerol alone (orange square). However, reporter expression in the ΔglpR or ΔglpR glpK22 background (pale blue diamonds and green triangles) responded more moderately and displayed a high background level.

d. Comparison between the flux relations of the ΔglpR strain (NQ958) and the glpR⁺ strain (NQ917) in the titratable LacY background. The threshold-linear shape is less prominent in ΔglpR (blue diamonds) than in glpR⁺ (purple circles). (Solid lines are guides to the eye.)

e. Intracellular G3P pool versus growth rate in the titratable LacY strain (NQ917) grown on lactose + glycerol with various 3MBA concentrations (purple circles), lactose alone with various 3MBA concentrations (green circles), glycerol alone without 3MBA (orange circle), and glucose alone without 3MBA (blue circle). The measurements for each mass spectrometry run were normalized by the result for growth on lactose alone with [3MBA] = 800 mM. In the presence of lactose + glycerol, the G3P pool abruptly drops if the growth rate exceeds that on glycerol alone.