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I am a biochem student and this has almost made me pull my hair out, here is the image of the question I got wrong, and I am not sure if the question is wrong or I am, I think the operator is downstream of the promoter in the sense/coding strand, right?

Question:

Where do you expect to find the lac operator in an expression vector for the E. coli system?

A: 3' (downstream) of the promoter region

B: 5' (upstream) of the promoter region

C: ORF

D: MCS

I chose A, but the "correct" answer is B, why? Does this differ from plasmid to plasmid? We study plasmids such as the pET-(24)+. Note that I am assuming they are talking about the sense/coding strand and not the template.

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The answer is B. The plasmid you mention itself supports it, and most other plasmids, too.

Do check back the mapping of your plasmid. The pET-24(+) plasmid you mentioned has the lac operator upstream of the lac promoter. As you can see here: It is upstream of promoter, downstream as blue, upstream as red

You can check the whole plasmid in SnapGene too.

Operon architecture is usually preserved on plasmids for the sake of convention.

Good luck!

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    $\begingroup$ The lac operator does not regulate the lac promoter in this expression system. It regulates the T7 promoter. The schematic shows the lac operator is downstream of the T7 promoter. $\endgroup$
    – canadianer
    May 3 at 13:16
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    $\begingroup$ @canadianer I have noticed so. Thank you for pointing it. Then it can be ambigious for the question asking a promoter. $\endgroup$
    – cc12amu
    May 3 at 20:43
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    $\begingroup$ I agree, it’s not a great question. $\endgroup$
    – canadianer
    May 3 at 21:32

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