Question

The ima operon of E. coli  controls "imaginary" biosynthesis (for this problem "imaginary" is a small vitamin that E.coli must biosynthesize or obtain from their environment in order to survive) . In addition to three structural genes (A, B, and C), there is also a regulatory region which we will call R for regulatory that is located at +15. Assays have been developed to independently monitor the activity of the protein products encoded by these genes. R- is a mutant regulatory region of the regulatory region consensus site has been altered from WT. The following data indicates the activity of the proteins encoded by the structural genes when imaginary is present or absent from the interior of the cell and with various genotypes of the regulatory region.  

1. Based on your knowledge of operons what is a likely mechanism for how the regulatory region of this operon (R) is helping to control transcription?

2. After a comprehensive search a protein has never been found that binds to the R regulatory region. By accident, however, it was found that imaginary can bind directly to the 5' end of the operon's mRNA. Propose a new mechanism for how the presence or absence of imaginary controls transcription.

3. If you monitored mRNA length in the presence or absence of imaginary what would you find?

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Answer 1

From the information given above, it can be concluded that the synthesis of the structural genes which are involved in the production of imaginary will be on only when its concentration is decreases or not present in the environment.

Question 1

As the regulatory sequence is present +15 from the transcription initiation site. therefore it is a downstream regulatory element. As shown in the figure, in the absence of imaginary, it might be possible that regulatory sequence bend to interact with the RNA polymerase. This interaction allows the stable binding of RNA polymerase with its promoter sequence and hence the transcription is ON. However, in the presence of imaginary, there will be no interaction of regulatory sequence with the RNA polymerases. This does not allow stable binding of RNA polymerase with the promoter and therefore the transcription is OFF. They're also more chances that the presence of imaginary will binds to the regulatory sequence and prevents its interaction with RNA polymerases.

Question 2

From the information given in the problem, it can be concluded that the presence of "imaginary" is inhibited its own synthesis. This will be done by binding of imaginary to the mRNA and either target this mRNA for the degradation. or not allowing the transition of mRNA.

Question 3.

As mentioned in problem 2, one of the possibilities of regulation of imaginary is to target mRNA bound imaginary for the degradation. Therefore it is expected that mRNA length would be shorter when the 'imaginary' is present and it would be longer in the absence of 'imaginary".