FIG. 7.

Two-step model for optimum induction kinetics of GAL1 transcription. Extracellular galactose is transported into the cytoplasm by Gal2p, and galactose-bound Gal3p or Gal1p antagonizes the Gal80 repressor, leading to the activation of the Gal4p activator and allowing it to recruit SWI/SNF. Chromatin remodeling by SWI/SNF leads to the rapid loss of promoter nucleosomes and PIC assembly, resulting in GAL1 induction. Gal4p activation (shown as Gal4p*) leads to an increased production of Gal1p and Gal3p, thereby amplifying the signal. k₁ and k₂ represent the rate-determining steps of this pathway, where k₁ reflects the action of Gal1p and Gal3p and k₂ reflects chromatin remodeling by SWI/SNF. In the absence of SWI/SNF, other nonspecific chromatin remodelers may cause promoter nucleosome loss. In the initial round of induction, Gal1p and Gal3p levels are low, making k₁ the rate-determining step. After the initial round of expression and a brief period of glucose repression, Gal3p and Gal1p levels are much higher than those in the initial state. Hence, k₁ is fast during GAL reinduction, making k₂ the slowest and rate-determining step.