The protein thorain is encoded by the gene thrL, and thorain is a negative regulator of the thrMN operon. Assume there is no other regulation taking place. Predict the phenotype of a thrL- mutant that encodes a nonfunctional thrL protein. a. The thrM and thrN genes would be transcribed at higher levels compared to non-mutant.
b. The thrM and thrN genes would be transcribed all lower levels compared to non-mutant.
c. The thrM and thrN genes would not be transcribed

The correct answer is option a, that is, the thrM and thrN would be transcribed at high levels in comparison to non-mutant.

Explanation:

Based on the given information, in the existence of thorain the expression of thrM and thrN will be prevented as thorain functions as a negative regulator. In case, if the gene, which codes for thorain, that is, thrL gets mutated, the coding for the functional thorain would get prevented.  

With the encoding of the non-functional thrL protein, there would be an enhancement in the expression of thrN and thrM genes and it will get transcribed at higher levels in comparison to the non-mutant or the wild-type.  

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