We did an experiment were we have e coli with a plasmid with a gene from another bacteria in it, and we put in IPTG in for induction. Will after looking up more about IPTG online I see it's related to the lac operator, which from what I've found just deals with lactose. Is there some other function that has? How can this be related to or effect the thing we put in and what we're doing? I'm missing the connection here.


1 Answer 1


The lac operon contains genes which are important for the metabolization of lactose as an energy source - normally glucose is used for this purpose. Usually the operon is tighly regulated and as long as there is another source of energy it is kept in an inhibited state.

The presence of lactose removes the lac repressor from the lac operon and allows the expression of the genes and thus allowing the metabolization of lactose. The mechanism can be turned on and off depending on the presence of lactose.

IPTG is a substance which mimicks the presence of allolactose (a metabolite of lactose) and it can activate transcription from the lac operon. As IPTG (in contrast to allolactose) cannot be hydrolyzed by β-galactosidase, it's concentration in the cell stays the same. Using the lac operon and IPTG enables you to switch on the expression of the gene on your plasmid and to start the overexpression.

  • $\begingroup$ So does the lac operon effect other things then lactose to that could have any affect on our protein? Or would there be something in the plasmid to let it be controlled by the lac operator? Is the presence or lack of the repressor having some effect? I mean what we're doing isn't related to lactose so I just don't get how messing with the lac repressor as an effect on what we put in the cell. $\endgroup$
    – windy401
    Feb 20, 2015 at 22:17
  • $\begingroup$ @windy401 In your plasmid (an artificial construct), the gene of interest will be under the control of the lac operator. Lac repressor can be expressed constitutively from another gene on the plasmid. $\endgroup$
    – canadianer
    Feb 20, 2015 at 23:24
  • $\begingroup$ Finally found this, and after reading it and the rest of the pages(mfa.od.ua/page274.htm), I get it now. "The essential element involved in lac-mediated protein expression is not the lac genes, but rather the lac regulatory apparatus. Generally, a gene selected for over-expression is cloned into a plasmid vector behind the lac promoter. Expression of this gene is then under lac control, either as an independent gene or fused to a fragment of lacZ or other genes." I just wasn't seeing how/why our gene was being controlled by something that just controlled lactose stuff. $\endgroup$
    – windy401
    Feb 21, 2015 at 2:55
  • $\begingroup$ You only use the promoter of the lac operon, sorry for being unclear here. This allows the control of the expression of your target gene. I will edit the answer later. $\endgroup$
    – Chris
    Feb 21, 2015 at 8:25

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