Question

Describe the process in which glucose affects the induction of ß-galactosidase by lactose.

Answer 1

Answer-

1. When glucose and lactose both are present in media then bacteria firstly use glucose as energy source as glucose enters bacterial metabolism directly. When glucose exhausted then metabolism shifts towards lactose.

2. When glucose concentration is low then it activates enzyme adenylate cyclase that synthesizes cAMP from ATP. The cAMP acts as trans factor for gene regulation. The cAMP binds to cAMP binding protein (CAP) and cAMP-CAP complex binds to CAP binding sequence (CBS) present very close to promoter. After binding CAP to lac promoter , RNA polymerase binds more efficiently to promoter and transcribes genes required for lactose metabolism.

3. But when glucose concentration is high, it inhibits adenylate cyclase enzyme that synthesizes cAMP from ATP. In absence of cAMP, CAP cannot bind to CBS site. In absence of CAP, RNA polymerase can not bind efficiently to promoter and genes for lactose utilization including beta galactosidase are not synthesized. Glucose also affects lactose transport inside cell by lactose permease by PEP group translocation. This mechanism works when phosphoenolpyruvate is present as energy source, and other sources like lactose are inhibited from entry. So that glucose affects the induction of beta galactosidase production in presence of lactose.