Here is the question:

Suppose an experimenter becomes proficient with a technique that allows her to move DNA sequences within a prokaryotic genome. If she moves the promoter for the lac operon to the region between the beta galactosidas gene and the permease gene which of the following would be likely?

Here is the answer:

Beta galactosidase will be produced.

My question to you is: why is this so? I have no idea where this conclusion came from. Thanks.

  • $\begingroup$ The question doesn't make any sense. Apparently before there is any manipulation there is already an intact lac operon present. Is it the lac operon or the lac operator that is being moved? And under what conditions is the production of beta-galactosidase being measured? $\endgroup$
    – Alan Boyd
    Feb 28, 2014 at 10:24
  • $\begingroup$ Unfortunately, I know nothing more about the question than you do. This is a test question that just doesn't make sense. $\endgroup$ Feb 28, 2014 at 10:27
  • $\begingroup$ If you can, I would ask the test writer to explain. $\endgroup$
    – jarlemag
    Feb 28, 2014 at 10:42

2 Answers 2


Since @biogirl has given an answer, I'll add my opinion:

β-galactosidase would be expressed but the permease and transacetylase would not.

The operator lies adjacent to/slightly overlaps the promoter, upstream of the lacZ gene. Binding of the repressor to the operator blocks the promoter, and induction of theoperon involves the repressor leaving the operator unoccupied. If you precisely excised the operator and transplanted it between the lacZ and lacY genes then I think the promoter would be constitutively expressed leading to β-galactosidase synthesis, but the operator would block the polymerase from transcribing the permease gene. In this view the operator would be acting more like a terminator.

Then, if an inducer was added it would relieve the effect of the transplanted operator and the downstream genes would be transcribed. It's even possible that this experiment has been done sometime in the last three decades.

added later:

The OP has now edited the question in such a way as to make my answer, and the answer from @biogirl, nonsensical. The original wording referred to moving "the lac operon" not "the lac promoter", hence my question in a comment asking "operon" or "operator"?

I'll leave my answer here, but this question is now so compromised that I advise everyone to just forget about it.

  • $\begingroup$ I'm inclined to agree with this answer. I know that the distinction between the lac operon being a distinct domain from the lac operon has been known since at least the early 1970s. The lac promotor/operon regulatory region is also known to be leaky, suggesting Alan's answer is correct. $\endgroup$
    – user560
    Mar 1, 2014 at 2:15

I believe the answer should be that permease is expressed. enter image description here

As you can see from the image, if the operator is moved between Z gene and Y gene, Y gene should be expressed and permease should be made.


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