Quantitative model for Gal4p-mediated expression of the galactose/melibiose regulon in Saccharomyces cerevisiae

A mathematical model based on equilibrium binding between Gal14p and its sp ecific DNA binding site has been developed. A model for GAL, gene expressio n solely due to cooperativity, as a function of Ga14p concentration, has be en developed for a gal80 mutant. The above model was extended to include ot her known regulatory molecules, namely Gal80p and Gal3p. Parameters determi ned from the above simulation were then used to represent a physiological s tatus of gene expression in response to glucose (in terms of Gal4p concentr ation) and galactose in a wild-type strain. We demonstrate that in a wild-t ype strain glucose repression is more stringent due to cooperativity and au togenous regulation, while the induction response to galactose is only thro ugh autogenous regulation. The biological significance of autogenous regula tion in Saccharomyces cerevisiae is discussed vis-a-vis the lactose operon of Escherichia coli.