Shown below is a diagram of the pxr operon, which consists of four hypothetical

Question

Shown below is a diagram of the pxr operon, which consists of four hypothetical genes labeled pxrA, pxrB, pxrC and pxrD. The pxrA, pxrB, and pxrD genes encode enzymes for the metabolism of a nutrient called NUX. Assume that in the absence of glucose, bacteria can grow in the presence of NUX alone. The pxrC gene encodes a gene regulatory protein that binds adjacent to the promoter of the pxr operon. To understand the regulatory properties of the PxrC protein, you engineer a mutant bacterium in which the pxrC gene has been rendered nonfunctional and look at the effects of this mutation on the transcription of the pxrA gene. If the PxrC protein works as a gene repressor, would you expect pxrA RNA levels to be high or low in the presence of NUX in the pxrC mutant cells? Briefly explain your answer. What about in the pxrC mutant cells in the absence of NUX? Briefly explain your answer. Your findings from the experiment are summarized in the following table: Do the results indicate that the PxrC protein regulates NUX metabolism by acting as a gene repressor or a gene activator? Briefly explain your answer.

Explanation / Answer

A: Since PxrC protein functions a s a repressor the levels of the PxrA RNA will be low in the presence of NUX in case of PxrC mutants. As a DNA-binding repressor blocks the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes into messenger RNAs including that of, PxrA RNA.

B: PxrC mutant cell lines in the absence of NUX there will be low level of RNAs of all the structural genes associated with the repressor. As NUX acts as a inducer and PxrC acts as a repressor .   

C: Thus based on the table given it can be concluded that Since PxrC protein functions a s a repressor as a gene activator as the level of PxrRNA in wild type strains with functional PxrC protein is high. Thus the facts in the table are different from the asumptions of the part a and b.

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