Timing and Variability of Galactose Metabolic Gene Activation Depend on the Rate of Environmental Change
Fig 6
An energy model for the glucose/galactose switch.
A) A schematic of the regulation in the heuristic stochastic model. Glucose inhibits expression of the GAL genes, but increases the cellular energy needed for protein production. Gal4p up-regulates transcription of gal2, whose gene product, Gal2p, imports galactose. The ability of the cell to metabolize galactose is therefore dependent on the availability of Gal2p. In addition, intercellular galactose increases cellular energy and up-regulates gal2. B) We used piecewise linear functions for Eglu([glu]) and Egal([Gal2p]). The Eglu term is assumed to only depend on the environmental glucose concentration. The Egal term does not depend on environmental galactose because it is held constant; what changes is the amount of galactose that is utilized by the cell, a good proxy for which is the concentration of Gal2p. The maximum (steady-state) energy levels and
are inferred from the cell-cycle lengths in the two conditions. The threshold thglu is the glucose threshold at which the cell is assumed to obtain maximal energy. The equivalent threshold for galactose is thgal; this is the threshold at which the cell has sufficient Gal2p to maximally utilize environmental galactose.