Practise AP Biology 4.2.B questions by tracing ligand recognition, receptor activation, intracellular amplification and cellular responses such as gene expression.
In eukaryotic microorganisms, the PHO signaling pathway regulates the expression of certain genes. These genes, Pho target genes, encode proteins involved in regulating phosphate homeostasis. When the level of extracellular inorganic phosphate (Pi) is high, a transcriptional activator Pho4 is phosphorylated by a complex of two proteins, Pho80-Pho85. As a result, the Pho target genes are not expressed. When the level of extracellular Pi is low, the activity of the Pho80-Pho85 complex is inhibited by another protein, Pho81, enabling Pho4 to induce the expression of these target genes. A simplified model of this pathway is shown in Figure 1.
Figure 1. A simplified model of the regulation of expression of Pho target genes in (A) a high-phosphate (high-Pi) environment and (B) a low-phosphate (low-Pi) environment
To study the role of the different proteins in the PHO pathway, researchers used a wild-type strain of yeast to create a strain with a mutant form of Pho81 (pho81mt) and a strain with a mutant form of Pho4 (pho4mt). In each of these mutant strains, researchers measured the activity of a particular enzyme, APase, which removes phosphates from its substrates and is encoded by PHO1, a Pho target gene (Table 1). They then determined the level of PHO1 mRNA relative to that of the wild-type yeast strain, which was set to 10.
TABLE 1. APase ACTIVITY AND RELATIVE AMOUNTS OF PHO1 mRNA IN WILD-TYPE AND MUTANT STRAINS OF YEAST IN HIGH- AND LOW-PHOSPHATE ENVIRONMENTS
Describe the effect that the addition of a charged phosphate group can have on a protein that would cause the protein to become inactive. Explain how a signal can be amplified during signal transduction in a pathway such as the PHO signaling pathway.
Based on Table 1, identify a dependent variable in the researchers' experiment. Justify the researchers' using the wild-type strain for the creation of the mutant strains. Justify the researchers' using mutant strains in which only a single component of the pathway was mutated in each strain.