Regulation of Gene Expression Flashcards
Which of the following mutations is most likely to result in reduced expression of the lac operon?
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.
Which of the following is best described as cis-acting?
The operator is part of the DNA itself, and so is cis-acting.
Which of the following is the basis for the intestine-specific expression of apolipoprotein B-48?
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.
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?
E. Inhibition of transcription of estrogen-responsive genes
Tamoxifen competes with estrogen for binding to the estrogen nuclear receptor. Tamoxifen fails to activate the receptor, preventing its binding to DNA sequences that upregulate expression of estrogen-responsive genes. Tamoxifen, then, blocks the growth-promoting effects of these genes and results in growth inhibition of estrogen-dependent breast cancer cells.
The ZYA region of the lac operon will be maximally expressed if:
D. the CAP site is occupied.
It is only when glucose is gone, cyclic adenosine monophosphate (cAMP) levels are increased, the cAMP–catabolite activator protein (CAP) complex is bound to the CAP site, and lactose is available that the operon is maximally expressed (induced). If glucose is present, the operon is off as a result of catabolite repression.
A research investigator is studying an autosomal recessive disease called I-cell disease (inherited lysosomal storage disorder). His field of study pertains to various protein targeting signals involved in protein trafficking. A precursor protein intended to function within a lysosome fails to receive a proper lysosomal tag while it is being processed. The protein will therefore be sent to which of the following sites?
Outside the cell – This is the correct answer. Lysosomal proteins are synthesized on bound ribosomes and the default pathway for them is to be secreted from the cell. If the mannose-6-phosphate lysosomal tag is not incorporated into an intended lysosomal precursor, then it will be sent outside the cell. TAKEAWAY: Protein trafficking is the transport of proteins to their correct subcellular compartments or to the extracellular space (“secretory pathway”). Endo- and exocytosis describe vesicle budding and fusion at the plasma membrane and are by most authors not included in the term protein trafficking.
