The term “inducible” promoter system comes into play when we look at the regulation of the LacI repressor protein. In fact, this idea initially originated from the lac operon. Let us take a moment to get re-acquainted with the lac operon:

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In these types of systems, an inducer – typically in the form of a chemical – is applied to the system. The purpose of the inducer is to bind to the LacI repressor protein and induce a conformation change in LacI such that it is released from the promoter. This means that the promoter site is now free and is recognized and bound by RNA polymerase and other transcription factors. The moral of this story is: if no LacI is bound to the promoter, transcription of your folA gene of interest can occur. 

So, to recap: in the absence of an inducer, the LacI protein is expressed and bound to the T7/lac promoter on the pET26b plasmid, therefore no DHFR protein expression occurs (1-2, 6). In chapters 1-5 you cloned your gene of interest downstream of this T7/lac promoter and so your DHFR proteins of interest will only be expressed once the LacI repressor protein is removed by the addition of an inducer (in your case the inducer molecule is called IPTG).

Now on to the special features of your E.coli BL21(DE3) cells which allow this specific inducible promoter system to work. Thus far we have largely ignored the T7 part of the T7/lac promoter name. We only looked at the regulation of the promoter itself, but this is just one of two important pieces of a promoter. The second piece is the DNA region of the promoter recognized by RNA polymerase. The endogenous E. coli RNA polymerase typically recognizes a specific DNA sequence in the promoter regions of all E. coli promoters. This makes sense as it needs to transcribe a whole bunch of genes, so having a similar recognition sequence in the promoter is efficient.  Now our promoter has a specific designation to it: T7. This promoter sequence was engineered specifically for your pET26b plasmid and was initially taken from a bacteriophage promoter sequence. A bacterio…what? Well, you can think of bacteriophages as bacterial viruses. These viruses (or phages as we like to call them in the bacterial world) infect bacteria specifically. Bet you never thought bacteria could get the flu! (nerd joke). Anyway, this is actually a really cool field because scientists have been studying these phages for a very long time. Actually, the techniques you are using in this course would not have been possible without understanding phages; and more importantly, how bacteria defend against phages. We visited one such defense mechanism in our discussion of restriction enzymes (RE). However, phages have also evolved their own mechanisms to manipulate bacteria. You must remember that phages (and viruses) infect a cell and then start manipulating the host cell’s machinery to make more copies of their own DNA and their own proteins. To do this, phages will often have their own RNA polymerase gene and, when expressed, the phage RNA polymerase will only recognize the phage promoter regions. So in our T7 expression system, we borrowed 2 items from a T7 bacteriophage: 1) a T7 promoter sequence which is recognized only by a T7 RNA polymerase, and 2) the T7 RNA polymerase gene. The T7 promoter sequence was genetically engineered into the pET26b plasmid as we saw previously.

The T7 RNA polymerase gene was genetically engineered into the chromosomal DNA of E. coli BL21(DE3) cells. This T7 RNA polymerase gene was inserted in the chromosomal DNA of this specific strain of E.coli, downstream of a lac promoter system. This lac promoter system is recognized by endogenous E.coli RNA polymerase and is also regulated by the LacI repressor in exactly the same fashion as described above.