The promoter of the araBAD operon from E. coli is activated in the presence of arabinose and the absence of glucose and directs expression of genes encoding enzymes required for arabinose metabolism. Unlike the cases of lac and gal genes, where a repressor and an activator work together, here two activators work together: AraC and CAP

+ arabinose	AraC binds that sugar and adopts a configuration that allows it to bind DNA as a dimer to the adjacent half-sites, araI1 and araI2
– arabinose	the AraC dimer adopts a different conformation and binds to araO2 and araI1, there is no monomer at site araI2, and so the protein cannot activate the araBAD promoter (araPBAD). This promoter is also controlled by CAP.

#Question id: 1200

#Unit 4. Cell Communication and Cell Signaling

Glucagon binds to the glucagon receptor, which interacts with a membrane-bound G protein, whose α subunit releases GDP and binds GTP. The α subunit then dissociates from the G protein and diffuses to a membrane-bound adenylyl cyclase which produces cAMP. In this scenario, name the

(1) transducer,

(2) effector,

(3) first messenger, and

(4) second messenger.

#Question id: 1201

#Unit 4. Cell Communication and Cell Signaling

Epinephrine binds to the same type of receptors in liver, fat, and smooth muscle cells. Yet in liver, glycogen breaks down; in fat, triacylglycerols break down; and smooth muscle cells relax. How can the same hormone produce three such different responses?

#Question id: 1202

#Unit 4. Cell Communication and Cell Signaling

G proteins are said to be self-inactivating. What G-protein function accounts for this?

#Question id: 1203

#Unit 4. Cell Communication and Cell Signaling

People who suffer from thyroid adenomas have constitutively activated G proteins in some cells of the thyroid gland. Tumor cells not only secrete copious quantities of thyroid hormones but also divide and grow excessively. Based on these observations, in what kind(s) of signal transduction pathways do the mutant G proteins participate?

#Question id: 1204

#Unit 4. Cell Communication and Cell Signaling

What factors are responsible for the specificity of G protein-coupled responses?

#Question id: 1205

#Unit 4. Cell Communication and Cell Signaling

Acetylcholine on a smooth muscle cell is to phospholipase C as epinephrine on a liver cell is to    ?    .