Models for Differential Gene Regulation
(A) Differential affinity of Pho4 for the PHO84 and PHO5 promoters can cause differential gene expression. Simulated curves of the percent occupancy of Pho4 at the PHO84 and PHO5 promoters, assuming Michaelian binding. Pho4 was modeled as having a Kd of 10 at the PHO84 promoter and of 100 at the PHO5 promoter. Phosphorylation of Pho4 was simulated as raising the Kd of Pho4 to 40 at the PHO84 promoter and to 400 at the PHO5 promoter. The nuclear concentration of Pho4 was assumed to be 7.5-fold higher in intermediate-phosphate medium than in high-phosphate medium and 10-fold higher in no-phosphate medium than in high-phosphate medium.
(B) Kinetic diagram of the steps leading to active transcription at the PHO84 and PHO5 promoters. Differences in the kinetic mechanisms of activation of PHO84 and PHO5 can lead to differential gene expression. Even if promoter occupancy is high at the PHO5 promoter, if the transcriptional activation step is slow compared to the rate of Pho4 phosphorylation and inactivation, PHO5 will not be induced.