Why would a green colony produce white colonies when streaked onto an LB/amp plate?

A green colony producing white colonies when streaked onto an LB/amp plate is primarily due to the absence of arabinose in the medium, which is required to induce the expression of the GFP gene. In many recombinant DNA systems, including those designed to produce GFP, the gene responsible for the protein's fluorescence is placed under the control of an inducible promoter. Often, this promoter can be activated by the presence of a specific sugar, such as arabinose.

When colonies are green, it indicates that they originally expressed the GFP due to the presence of arabinose in the previous medium used for growth. However, when these green colonies are plated on an LB/amp plate that does not contain arabinose, the promoter that drives the expression of GFP is inactive. Consequently, the cells do not produce the GFP protein, which means they will not fluoresce green when observed under UV light. The lack of arabinose results in the expression of other genetic characteristics, which can lead to the appearance of white colonies.

Thus, the presence of arabinose is crucial for maintaining the green fluorescence associated with GFP, and its absence can result in the loss of this trait and the appearance of white colonies.