Models for Differential Gene Regulation

<div><p>(A) Differential affinity of Pho4 for the <i>PHO84</i> and <i>PHO5</i> promoters can cause differential gene expression. Simulated curves of the percent occupancy of Pho4 at the <i>PHO84</i> and <i>PHO5</i> promoters, assuming Michaelian binding. Pho4 was modeled as having a <i>K</i><sub>d</sub> of 10 at the <i>PHO84</i> promoter and of 100 at the <i>PHO5</i> promoter. Phosphorylation of Pho4 was simulated as raising the <i>K</i><sub>d</sub> of Pho4 to 40 at the <i>PHO84</i> promoter and to 400 at the <i>PHO5</i> 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.</p> <p>(B) Kinetic diagram of the steps leading to active transcription at the <i>PHO84</i> and <i>PHO5</i> promoters. Differences in the kinetic mechanisms of activation of <i>PHO84</i> and <i>PHO5</i> can lead to differential gene expression. Even if promoter occupancy is high at the <i>PHO5</i> promoter, if the transcriptional activation step is slow compared to the rate of Pho4 phosphorylation and inactivation, <i>PHO5</i> will not be induced.</p></div>