2023 AP Biology Free-Response Questions

1. 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 Yeast Strain Mutation APase Activity in High-Pi Environment (mU/mL/OD600) ±2SE x̄ APase Activity in Low-Pi Environment (mU/mL/OD600) ±2SE x̄ Relative Amounts of PHO1 mRNA in High-Pi Environment ±2SE x̄ Relative Amounts of PHO1 mRNA in Low-Pi Environment ±2SE x̄ Wild-type None 0.5 ± 0.1 17.3 ± 0.9 0.1 ± 0.0 10 ± 2.0 pho81mt Nonfunctional Pho81 0.4 ± 0.1 0.6 ± 0.1 0.7 ± 0.2 0.9 ± 0.8 pho4mt Nonfunctional Pho4 0.5 ± 0.0 0.8 ± 0.2 0.6 ± 0.4 0.3 ± 0.1 (a) 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. (b) 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. (c) Based on the data in Table 1, identify the yeast strain and growth conditions that lead to the highest relative amount of PHO1 mRNA. Calculate the percent change in APase activity in wild-type yeast cells in a high-Pi environment compared with that of wild-type cells in a low-Pi environment. (d) In a follow-up experiment, researchers created a strain of yeast with a mutation that resulted in a nonfunctional Pho85 protein. Based on Figure 1, predict the effects of this mutation on PHO1 expression in the mutant strain in a high-Pi environment. Provide reasoning to justify your prediction. Write your responses to this question only on the designated pages in the separate Free Response booklet.