Question

James is working with the lac operon of Escherichia coli (E. coli). He places the bacteria on a plate of growth media.

The lac Operon of E. coli is shown.

Based on the current understanding of this operon, which hypothesis would be useful for James to test?
Addition of allolactose (inducer) to the bacterial growth media should increase the speed at which the bacteria metabolize the sugar lactose.
Removal of the RNA polymerase molecule should increase the amount of bacterial growth on the plate.
Decreasing the amount of allolactose (inducer) in the bacterial growth media should increase the rate of bacterial growth.
Increasing the rate at which RNA polymerase acts will inhibit bacterial growth.

Answer 1

Addition of allolactose (inducer) to the bacterial growth media should increase the speed at which the bacteria metabolize the sugar lactose.

Rationale: allolactose binds and inactivates the lac repressor, allowing RNA polymerase to transcribe the lacZYA genes that encode β‑galactosidase and permease. This increases lactose uptake and breakdown, so adding inducer should increase lactose metabolism (and growth on lactose medium). The other choices contradict basic lac operon function (removing RNA polymerase would stop transcription; reducing inducer lowers expression; boosting RNAP activity would not be expected to inhibit growth).