Why are the colonies red on the LB/amp/ara plate but not on the LB/amp plate?

Why are the colonies red on the LB/amp/ara plate but not on the LB/amp plate?

• A. Arabinose enables expression of red fluorescent protein via araC and Pbad.
• B. Ampicillin is required for red pigment synthesis.
• C. Red pigment is produced only when lactose is present.
• D. Red color is inherent to transformed colonies regardless of media.

Answer: (A)

Arabinose acts as an inducer for the araC-Pbad system. When arabinose is present, AraC changes shape and activates transcription from the Pbad promoter, turning on the expression of the red pigment gene. That makes the colonies appear red on the LB/amp/ara plate. In contrast, without arabinose, this promoter stays off (AraC represses it), so the pigment isn’t produced and the colonies aren’t red. Ampicillin is only there to select for transformed cells in both plates; it doesn’t drive pigment production.

Arabinose acts as an inducer for the araC-Pbad system. When arabinose is present, AraC changes shape and activates transcription from the Pbad promoter, turning on the expression of the red pigment gene. That makes the colonies appear red on the LB/amp/ara plate. In contrast, without arabinose, this promoter stays off (AraC represses it), so the pigment isn’t produced and the colonies aren’t red. Ampicillin is only there to select for transformed cells in both plates; it doesn’t drive pigment production.