4. You are interested in studying the transcriptional regulation of a gene expre

Question

4. You are interested in studying the transcriptional regulation of a gene expressed in the gut called Smrtl. The promoter for Smrtl contains binding sites for two transcriptional regulators called A and B. Proteins A and B bind to the promoter by recognizing specific sequences called bindin;g sites. The binding sites for these proteins are diagrammed in the figure. As shown, the binding sites for protein A and B overlap; it has beern demonstrated that proteins A and B cannot bind simultaneously. It has also been demonstrated Experiment Binding siteGlucose Smr mRNA that protein A is an allosteric protein that is number: AB regulated by the sugar glucose, such that A can 1 only bind to DNA when glucose is present. Yoiu make changes in the binding sites such that the protein cannot bind to these specific sites 5 (indicated by a "-" in the table; a "+" indicates 6 that the protein can bind) and measure Smrtl 7 mRNA production in cells containing equal L8 amounts of A and B proteins Your results are summarized in the Table. Smrt1 transcription start site resent? duction? no ves no ves es no 4 es es no no no no no es no es i) If the top strand of the DNA in the Figure is written in a 5' to 3' orientation, is the top or the bottom strand the template for transcription? (2 points) i) How does the binding of protein A affect gene expression? (3 points) ii) How does the binding of protein B affect gene expression? (3 points) iii) In experiment number 1, which protein is bound to the Smrtl promoter? (3 points) iv) Which protein, A or B, binds to its binding site more tightly? Explain. Be sure to state which experiments above support your conclusion. (7 points)

Explanation / Answer

Since the transcription always occurs from 5’ to 3’, and the direction of transcription is given (from left to right), the template strand is 3’ to 5’ from left to right.

Since the top strand is in 5’ to 3’ orientation, the bottom strand has to be in 3’ to 5’ orientation. Hence the bottom strand acts as template strand.

Protein A binds only in the presence of glucose. In the given table, when glucose is present and when the protein A is able to bind, there is no production of mRNA. It indicates that protein A represses the transcription.

In the table, when protein A is unable to bind and when protein B is able to bind (1, 3, 4), mRNA production is observed. Therefore, protein B activates the gene expression.

In experiment 1, since glucose is not present, protein A cannot bind to its binding site. Protein B, therefore, binds to the promoter.

Protein A has stronger affinity to the promoter than protein B. in experiment 2, both the proteins are able to bind the promoter. Even though, the transcription is repressed. It indicates repressor has more affinity to the promoter than the activator. Therefore, protein A (in the presence of glucose) has a stronger affinity to the promoter.

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