Why did red colonies appear only on LB/amp/ara plates and not on LB/amp plates?

Why did red colonies appear only on LB/amp/ara plates and not on LB/amp plates?

• A. Because ampicillin is present on LB/amp plate.
• B. Because arabinose is required to express the rfp gene; only on LB/amp/ara plate shows red colonies.
• C. Because plasmid replication cannot occur on LB/amp plate.
• D. Because red pigment is produced without arabinose.

Answer: (B)

Red colonies appear only when arabinose is present because the red pigment gene is controlled by an arabinose-inducible promoter. Without arabinose, AraC keeps the promoter off, so the RFP gene isn’t expressed and no red pigment is produced, even though the plasmid (which also carries ampicillin resistance) is present. Ampicillin just selects for cells with the plasmid, so growth occurs on both plates. When arabinose is included, it activates the promoter via AraC, turning on RFP expression and producing the red colonies.

Red colonies appear only when arabinose is present because the red pigment gene is controlled by an arabinose-inducible promoter. Without arabinose, AraC keeps the promoter off, so the RFP gene isn’t expressed and no red pigment is produced, even though the plasmid (which also carries ampicillin resistance) is present. Ampicillin just selects for cells with the plasmid, so growth occurs on both plates. When arabinose is included, it activates the promoter via AraC, turning on RFP expression and producing the red colonies.