Question

2. In the facultatively anaerobic bacterium, Klebsiella pneumoniae, the presence of O2 inhibits Nar and Frd gene expression. However, the absence of O2 and presence of NO3 activates Nar and inhibits Frd gene expression. Only when both O2 and NO3 are absent does Frd genes express. Explain why this occurs.

Answer 1

Clearly nar (nitrate reductase) gene and frd (fumarate reductase )gene do not require oxygen for its expression. Both these genes express during anaerobic respiration, and both the enzymes help reduce other compunds , not oxygen.  Presence of nitrate activates the nar gene. Nitrate reductase converts nitrate to nitrite. In the absence of both oxygen and nitrate frd gene is activated. Fumarate reductase is the enzyme that converts fumarate to succinate.