My textbook states that

lactose permease...transports lactose into the cell


When lactose is added to the growth medium, the lactose molecules bind to the other site on the repressor protein

My question is, how can lactose enter the cell to bind to the repressor protein to allow beta galactosidase and lactose permease to be synthesised if there is no lactose permease present to transport the lactose into the cell?

  • $\begingroup$ Based on the questions you are asking, you have good intuition about the information you are reading. Out of necessity introductory textbooks are cursory and only cover the very basics of only the most important of topics. They basically are teaching you the vocabulary of biology and keeping the detail at arms length. Keep reading and keep looking deeper, you are on the right track. $\endgroup$
    – AMR
    Nov 22, 2015 at 13:46
  • $\begingroup$ Here's everything you need. It's called the lactose paradox: sandwalk.blogspot.it/2008/10/lactose-paradox.html $\endgroup$ Jun 5, 2016 at 15:45

1 Answer 1


Introductory textbooks will not get into the details of the lac operon.

Basically, the operon is expressed constitutively at a low level that means that Beta Galactosidase and Lactose Permease are expressed at low levels by the bacterium. This is because it takes a little bit of time to build up the concentration of LacI in the cell before it can start to bind the operator.

The LacI repressor only binds the operator in the absence of Lactose in the cell's environment due to the fact that allolactose allosterically binds the LacI repressor and affects that molecule's ability to bind the operator. So RNA polymerase can bind the promoter of the Lac operon at some low rate and continue to produce the protein.

When the cell no longer has glucose, it generates cyclic AMP, which induces the CAP protein to bind to the CAP site, just upstream of the promoter of the Lac Operon. CAP associates with RNA Polymerase, and this makes the promoter very active. Beta Galactosidase and Lactose Permease levels increase rapidly.

Where you see the full effects of the LacI repressor is when neither Glucose nor Lactose are present in the environment of the cell. At this point the repressor binds to the operator and blocks transcription by DNA Polymerase.

I hope this helps. I have an entire book that covers the Lac Operon, so even though this is a bit expanded of an explanation from a basic textbook, it is still a summary.

  • $\begingroup$ Thank you for your informative reply! I'm just still slightly confused; by 'the operon is expressed constitutive lay at a low level' do you mean that even with no lactose present, a repress or protein is not constantly bound and therefore some beta galactosidase and lactose permease are expressed? This seems to contradict with 'when neither Glucose nor Lactose are present in the environment of the cell...the repress or bonds to the operator and blocks transcription by RNA polymerase'. I apologise for misunderstanding and being quite slow with this, and thank you again for your help :) $\endgroup$
    – Meep
    Nov 23, 2015 at 14:11
  • 1
    $\begingroup$ @21joanna12 it is going to take me a bit of time to update my answers. But very briefly, DNA binding proteins all have a certain on/off rate associated with them. There will be certain instances where the operator is clear and RNA Pol can bind. Also proteins have half-lives and are degraded, so depending on the cytoplasmic concentration, there is a probability that the promoter will be free to be bound by RNA Pol. Cell biology is unfortunately not digital as it is often described in textbooks. I'll try to get to a better answer soon. $\endgroup$
    – AMR
    Nov 23, 2015 at 21:39

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