Question

The FepC, D and G are proteins of E. coli are required to transport Fe-siderophore complexes from the periplasm across the cytoplasmic membrane into the cytoplasm. In order to first transport these complexes from the environment into the periplasm, the outer membrane protein FepA is required. Expression of the fepA gene is regulated in the same way as expression of the genes in the fepCDG operon. Given this information, which one of the following statements is correct?

A. In the absence of sufficient iron, a Fur-FepA complex spends most of its time bound to the fur box.

B. In the presence of sufficient iron, Fur spends most of its time bound to the fepA promoter.

C. In the absence of sufficient iron, RNA polymerase is frequently blocked from accessing the fepA promoter.

D. In the presence of sufficient iron, the Fur-Fe complex spends most of its time bound to the fepA fur box.

Answer 1

Answer: D. In the presence of sufficient iron, the Fur-Fe complex spends most of its time bound to the fepA fur box. (When Fe is sufficiently present, it binds to Fur and makes the Fur-Fe complex, which has higher affinity for the operator region, i.e. fur box of fepA gene.)

Option A is wrong as in absence of iron, Fur-Fe complex releases Fe. The Fur protein alone doesn't have much affinity for fur box, leaving it clear, which leads to enhanced transcription from the promoter.

Option B is wrong as Fur-Fe binds to fur box, the operator and not the promoter itself. RNA polymerase binds to the promoter.

Option C is wrong as in absence of iron, with Fur-Fe not bound to operator, RNA polymerase easily accesses the fepA promoter.