C. Oc (operator cannot bind repressor protein)
D. One resulting in impaired glucose uptake
Correct answer = A. In the absence of glucose, adenylyl cyclase makes cyclic adenosine monophosphate (cAMP), which forms a complex with the catabolite activator protein (CAP). The cAMP–CAP complex binds the CAP site on the DNA, causing RNA polymerase to bind more efficiently to the lac operon promoter, thereby increasing expression of the operon. With cya− mutations, adenylyl cyclase is not made, and so the operon is unable to be maximally expressed even when glucose is absent and lactose is present. The absence of a repressor protein or decreased ability of the repressor to bind the operator results in constitutive (essentially constant) expression of the lac operon.
3.2. Which of the following is best described as cis-acting?
A. Cyclic adenosine monophosphate response element–binding protein
B. Operator
C. Repressor protein
D. Thyroid hormone nuclear receptor
Correct answer = B. The operator is part of the DNA itself, and so is cis-acting. The cyclic adenosine monophosphate response element–binding protein, repressor protein, and thyroid hormone nuclear receptor protein are molecules that diffuse (transit) to the DNA, bind, and affect the expression of that DNA and so are trans-acting.
3.3. Which of the following is the basis for the intestine-specific expression of apolipoprotein B-48?
A. DNA rearrangement and loss
B. DNA transposition
C. RNA alternative splicing
D. RNA editing
E. RNA interference
Correct answer = D. The production of apolipoprotein (apo) B-48 in the intestine and apo B-100 in liver is the result of RNA editing in the intestine, where a sense codon is changed to a nonsense codon by posttranscriptional deamination of cytosine to uracil. DNA rearrangement and transposition, as well as RNA interference and alternative splicing, do alter gene expression but are not the basis of apo B-48 tissue-specific production.
3.4. Which of the following is most likely to be true in hemochromatosis, a disease of iron accumulation?
A. The messenger RNA for the transferrin receptor is stabilized by the binding of iron regulatory proteins to its 3′-iron-responsive elements.
B. The messenger RNA for the transferrin receptor is not bound by iron regulatory proteins and is degraded.
C. The messenger RNA for ferritin is not bound by iron regulatory proteins at its 5′-iron-responsive element and is translated.
D. The messenger RNA for ferritin is bound by iron regulatory proteins and is not translated.
E. Both B and C are correct.
Correct answer = E. When iron levels in the body are high, as is seen with hemochromatosis, there is increased synthesis of the iron-storage molecule, ferritin, and decreased synthesis of the transferrin receptor (TfR) that mediates iron uptake by cells. These effects are the result of cis-acting iron-responsive elements not being bound by trans-acting iron regulatory proteins, resulting in degradation of the messenger RNA (mRNA) for TfR and increased translation of the mRNA for ferritin.
3.5. Patients with estrogen receptor–positive (hormone responsive) breast cancer may be treated with the drug tamoxifen, which binds the estrogen nuclear receptor without activating it. Which of the following is the most logical outcome of tamoxifen use?
A. Increased acetylation of estrogen-responsive genes
B. Increased growth of estrogen receptor–positive breast cancer cells
C. Increased production of cyclic adenosine monophosphate
D. Inhibition of the estrogen operon
E. Inhibition of transcription of estrogen-responsive genes