Dual feedback loops in the GAL regulon suppress cellular heterogeneity in yeast Supplementary Note Stephen Ramsey, Jennifer J. Smith, David Orrell, Marcello Marelli, Timothy W. Petersen, Pedro de Atauri, Hamid Bolouri * , and John D. Aitchison † Institute for Systems Biology 1441 N 34th St. Seattle, WA 98103 July 27, 2007 Model Description In this section we briefly present our mathematical model of the yeast galac- tose utilization pathway. This model is based on an earlier published model [1], with some simplifications. The simulation results in this study were obtained using the chemical kinetics modeling program Dizzy [2]. This soft- ware is available (under a free and open-source license) on the World Wide Web at http://magnet.systemsbiology.net/software/Dizzy. The species names in the model are defined in Table 1. In addition to the dynamical symbols defined in Table 1, the symbol G ex (shown as GAE in the text) is used to denote the concentration of extracellular galactose. G ex is a free parameter in the model. A species symbol appearing in the kinetic equations of the model represents a cell population average number of molecules per unit volume, where the unit of volume is 3.57 × 10 -14 liters (a typical cell volume for haploid yeast [1]). Similarly, “molec” appearing as a unit in the kinetic constants for the model represents molecules per unit volume as defined above. [The exception to this is the case of Gal80p, for which the number of molecules in the cytoplasm and in the nucleus are * hbolouri@systemsbiology.org † jaitchison@systemsbiology.org 1