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Q27 - Gene Regulation by Protein Y

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Gene Regulation: Eukaryotic & Epigenetics

The regulatory region of a bacterial gene, called ‘A’, is shown below. The promoter (P) and regulatory region (R) regulate the transcription of the gene.

Chemical X regulates the expression of the gene by interacting with a protein Y. There are 3 binding sites (R1, R2, R3) for protein Y in region R. Protein Y has different binding affinities for the 3 sites.

Gene A regulatory region showing promoter P and binding sites R1, R2, R3

An experiment was carried out to study the role of the 3 binding sites. The binding sites (R1 to R3) were deleted individually, or in combination, and the expression of gene ‘A’ was measured in the presence or absence of chemical X.

In each of the strains below, the expression of gene ‘A’ in the absence of chemical ‘X’ is 15 arbitrary units.

Binding site deletion experiment results table

Strain R1 R2 R3 Ratio (presence:absence of X)
WT ✓ ✓ ✓ 120
M1 ✓ ✓ ✗ 52
M2 ✓ ✗ ✓ 3
M3 ✗ ✓ ✓ 107
M4 ✗ ✓ ✗ 30
M5 ✓ ✗ ✗ 1
M6 ✗ ✗ ✓ 1
M7 ✗ ✗ ✗ 1

True or false?

Q27.1. If gene Y is deleted, the ratio of activity will increase.
Q27.2. The binding affinity of protein Y for R2 site is expected to be the highest.
Q27.3. The sequence TGTGA is important for the binding of protein Y to the R sites.
Q27.4. The binding of protein Y to R3 and R2 influences the gene activity in the same way as protein Y binding to R1 and R2.
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Question reproduced from IBO 2023, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF