Under which condition is lacZYA transcription maximal?

Under which condition is lacZYA transcription maximal?

• A. Lactose absent; glucose present.
• B. Lactose present; glucose present.
• C. Lactose present; glucose low.
• D. Lactose absent; glucose low.

Answer: (C)

Maximal lacZYA transcription occurs when the operon is both derepressed and strongly activated. Lactose must be present so allolactose binds the LacI repressor and releases it from the operator, removing the block on transcription. At the same time, glucose must be low because low glucose raises cAMP levels, allowing CAP to bind promoter and greatly boost RNA polymerase recruitment. When both conditions are met—lactose present and glucose low—the combination of derepression and CAP activation yields the highest transcription. If lactose is absent, the repressor remains bound and transcription stays low. If lactose is present but glucose is high, CAP is not activated and transcription is not maximal. If lactose is absent and glucose is low, CAP is activated but repression still limits transcription, so it’s not maximal. Therefore, lactose present with glucose low is the condition that gives the strongest expression of lacZYA.

Maximal lacZYA transcription occurs when the operon is both derepressed and strongly activated. Lactose must be present so allolactose binds the LacI repressor and releases it from the operator, removing the block on transcription. At the same time, glucose must be low because low glucose raises cAMP levels, allowing CAP to bind promoter and greatly boost RNA polymerase recruitment. When both conditions are met—lactose present and glucose low—the combination of derepression and CAP activation yields the highest transcription. If lactose is absent, the repressor remains bound and transcription stays low. If lactose is present but glucose is high, CAP is not activated and transcription is not maximal. If lactose is absent and glucose is low, CAP is activated but repression still limits transcription, so it’s not maximal. Therefore, lactose present with glucose low is the condition that gives the strongest expression of lacZYA.