Question

The gene REG encodes an activator protein (REG). REG is a transcriptional activator involved in the regulation of the gene YLW. You are studying a species of flowers where the YLW protein is involved in flower petal colour.

You have generated a number of mutant flower lines, and are measuring the amount of REG transcription that occurs in each line. The results are below:

Note: The results are given relative to wild-type/unmutated (e.g., a transcription level of 100% in a mutant would mean transcription levels equal to wild-type; a transcription level of 20% in a mutant would mean the mutant has transcription levels that are 20% of wild-type levels, and so on).

REG transcript levels (relative to wild-type/unmutated)

• Mutant #1 - 35%
• Mutant #2 - 0%
• Mutant #3 - 175%
• Mutant #4 - 200%

A) Given the transcript levels above, what type of cis-element is likely damaged in each mutant (assume the mutation makes the cis-element stop working; the mutation rate is such that you'd only expect one element to be effected per mutant and everything else in the genome remains unmutated)? To help you answer, your answer will come from one of the following: silencer, enhancer, core/proximal promoter, activator, repressor.

B) For each mutant, what would you expect the level of YLW protein to be relative to wild-type/unmutated? Use "more than wild type", "less than wild type", "same as wild type".

Answer 1

A) The mutation is being caused in the cis elements. The cis elements in the gene are silencer, enhancer, activator, repressor and core/proximal promoter. The silencer is the gene regulatory element that keeps the transcription in check if no activator is present by activating the repressor. The enhancer is the gene regulatory element that keeps the transcription active when activator is present by removing or inhibiting the repressor. The promoter is the actual place for the transcription start site. In mutant 1, the enhancer is mutated thereby it only activates the promoter to a certain extent and hence only 35% of the transcript is seen. In mutant 2, the promoter is being mutated which is the key part that initiates transcription. If the promoter is mutated, then there is no binding of polymerase and hence there is no transcription occuring. That is the reason for 0% transcript levels in mutant 2. In mutant 3 and mutant 4, the silencer is being mutated and hence overexpression of transcript is taking place.

B) In mutant 1, the level of YLW protein will be less than the wild type protein. In mutant 2, the level of YLW protein will be less than wild type protein. In mutant 3, the level of YLW protein will be more than the wild type protein. In mutant 4, the level of YLW protein will be more than the wild type protein.

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