Question

You are interested in understanding the gene regulation of Lkp1, a protein that is normally produced in liver and kidney cells in mice. You find that LKP1 gene is not expressed in heart cells. You isolate the DNA upstream of the gene, and place it upstream of the gene for GFP and insert this entire piece of recombinant DNA into mice. You find GFP expressed in liver and kidney cells. Experiments demonstrate that there are three regions in the promoter labeled, A, B, and C that contribute to this expression pattern. Assume that a single transcription factor binds each site such that protein X binds site A, protein Y binds site B, and protein Z binds site C. Determine which region is responsible for tissue-specific expression and what the function of each region is. If the site is missing, it is mutated such that it cannot bind its corresponding transcription factor.

Which of the following proteins is likely to act as a GENE REPRESSOR?
(a) factor X
(b) factor Y
(c) factor Z
(d) none

Answer 1

In order to identify which transcription factor acts as a gene repressor, we would need information on the effects of mutations at the binding sites A, B, and C. A repressor would normally prevent gene expression in the tissue where the gene is not usually expressed, and its absence (due to mutation) would lead to expression in that tissue. Without specific data on the mutations, we cannot definitively identify which transcription factor functions as a repressor.

Step-by-step explanation:

To understand which region is responsible for the tissue-specific expression of the LKP1 gene, and thus the expression of GFP in liver and kidney cells but not heart cells, we need to determine the role of each of the three regions A, B, and C, and their associated transcription factors X, Y, and Z. Since the GFP is expressed only in liver and kidney cells, and the question suggests that the binding of each transcription factor to its respective site contributes to this expression pattern, if the site is mutated, it means that the corresponding transcription factor cannot exert its effect. Therefore, the transcription factor that acts as a gene repressor would be the one associated with a site that, when mutated, leads to GFP expression in cells where it was not expressed before (in this case, possibly the heart cells).