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This is the actual question. It's from an entrance test for a research institute:

A scientist wants to express human protein Y in bacteria. For effective expression of this protein he should use (A) promoter of human gene Y. (I think this is the correct one) (B) promoter of bacterial gene. (C) operator of any human gene. (D) operator of any bacterial gene.

I think my answer is right although I am not sure if my thinking is entirely correct. enter image description here

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  • $\begingroup$ Homework questions are not allowed unless you show your attempt at an answer, so please share your thinking. $\endgroup$ – canadianer Jun 21 '18 at 4:19
  • $\begingroup$ I am pretty sure there is no way to convince you but this is not a homework question. It's from an entrance test for a research institute in my country. $\endgroup$ – Fenil Jun 21 '18 at 4:28
  • $\begingroup$ I believe you, though some level of effort is expected for all questions. Here's a leading question: why would bacterial transcription factors recognize a human promoter? $\endgroup$ – canadianer Jun 21 '18 at 4:33
  • $\begingroup$ My guess is that they probably wouldn't? Instead the prokaryotic RNA polymerase is more likely to recognise a bacterial promoter. But I don't know if a bacterial promoter could work with a human gene, hence the confusion. $\endgroup$ – Fenil Jun 21 '18 at 4:51
  • $\begingroup$ Yes, that's correct. The promoter essentially directs RNA polymerase to start transcription at a specific point, but transcription initiation isn't generally concerned with what comes after the transcription start site. Bacterial promoters (and viral promoters, which have coevolved to be recognized by bacterial transcription factors) are used frequently for heterologous protein expression. $\endgroup$ – canadianer Jun 21 '18 at 5:07
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Canadianer has very deftly enabled me to arrive at the answer with knowledge I already had. I attempt to explain the same here.

We know that eukaryotes and prokaryotes have different RNA polymerase (enzyme responsible for transcription) enzymes. Eukaryotes have three different RNA polymerase, each responsible for synthesis of the three different types of RNAs. RNA Polymerase II is responsible for mRNA synthesis in eukaryotes. It recognises the eukaryotic (or say, human) promoter present in an eukaryotic cell. On the other hand, in prokaryotes, there's only one RNA polymerase responsible for synthesis of all the types of RNA.

Coming to the question, using a human promoter is probably logical since it would be ideal for initiation of transcription of the said human gene Y. But this is not true because a promoter is only responsible for recognition by RNA polymerase and allowing it to move further and transcribe the DNA sequence i.e. it doesn't matter what comes after a promoter. Also, prokaryotic RNA polymerase is unlikely to recognise the sequence of a human promoter. It is much more likely to recognise a bacterial promoter. Since a promoter's function is limited to allow recognition, it doesn't matter that it's a bacterial promoter as long as it can be recognised.

The two options about operators weren't a confusion for me since I already know that they don't have much to do with expression of gene as much as regulating it. Besides, any operator used would also require a promoter in any case, since that is how transcription is initiated.

Thus, the scientist should use a bacterial promoter.

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    $\begingroup$ You're welcome. Credit goes to you. $\endgroup$ – Fenil Jun 22 '18 at 1:53

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