Question

Which of these is NOT true about the Lac Operon?

Group of answer choices

Uses an activator to ensure lactose is present and a repressor to ensure glucose is absent

Uses both an activator and repressor to ensure proper conditions are proper for activation of the operon

Makes enzymes that allow bacteria to use lactose as a source of energy

Uses an activator to ensure glucose is absent and a repressor to ensure lactose is present

Answer 1

The correct answer is statement 1 (Uses an activator to ensure lactose is present and a repressor to ensure glucose is absent) because it is an incorrect statement.

Statement 4 (which is opposite of statement 1) is correct. (please see explanation below to understand why statement 1 is incorrect and statement 4 is correct)

Lac operon is controlled by two mechanisms:

• Induction
• catabolite repression

During Induction, three structural genes of Lac operon is controlled by a diffusible product of LacI gene (repressor). Repressor remain bounded to the operator and block the transcription. Lactose (more specifically allolactose) is the inducer of lac operon and when lactose is present, allolactose is formed and binds with the repressor and make the operator free for RNA polymerase to bind. Hence Lac operon uses a repressor to ensure lactose is present.

Another way of regulation of lac operon is catabolite repression where presence of glucose, lac operon is repressed. This is mediated by

• Catabolite activator protein (CAP) (an activator)
• cAMP

The promoter of Lac operon has binding site for RNA polymerase as well as for CAP-cAMP complex (activator complex) and this complex is required for stimulation of RNA polymerase.

When glucose is not present (it means cell has to utilize lactose as energy source), CAP-cAMP activator complex remain bounded to the Lac promoter and transcription continues. However when glucose is present it reduces cAMP and the activator CAP become inactive and can not bind to Lac promoter and transcription stops. Hence it suggests that Lac operon uses activator to ensure that glucose is absent.

Statement 2 is correct. As explained above Lac operon is controlled by both activator and repressor.

Statement 3 is correct. Lac operon has three structural genes

• LacZ: encodes beta-galactosidase (breaks lactose in to glucose and galactose)
• LacY: encodes beta-galactoside permiase (transport beta-galactosides in to cells)
• LacA: encodes beta-galactoside transacetylase (modify toxic galactoside in order to remove them from the cell)

These enzymes ensure bacteria to use lactose as source of energy.