Imagine a bistable system composed of two repressor proteins X and Y, each of which binds to the cis-regulatory sequences of the gene encoding the other protein, i.e. X represses the expression of Y, and Y represses the expression of X. A researcher engineers the genes such that they encode protein X fused to green fluorescent protein (GFP) and protein Y fused to red fluorescent protein (RFP). Cells containing the fusion constructs are then grown and sorted by fluorescence-activated cell sorting (FACS) based on their GFP and RFP signals. The results are presented in the following dot plot. Each cell is represented by a dot whose coordinates correspond to GFP and RFP fluorescence in the cell. According to these results, which protein (X or Y) do you think binds to its target DNA sequence with a higher degree of cooperativity? Write down X or Y as your answer.

Question

Quizplus · Accepted Answer

As expected, most cells express either high levels of X-GFP or high levels of Y-RFP but not both. The switchlike behavior is sharper for X-GFP: there are almost no cells with intermediate GFP signal intensity, whereas many cells show an intermediate RFP signal. This suggests that binding of Y to its gene may occur with a relatively higher cooperativity.

Imagine a Bistable System Composed of Two Repressor Proteins X