This is Darrell Killian, Associate Professor of Molecular Biology at Colorado College and one of the authors of your textbook.
In this walkthrough, we will examine the regulation of gene expression in bacteria using the lac operon as an example.
Let's start by discussing the components of the system.
Recall that an operon is a cluster of genes under the control of a common regulatory region.
The lac operon has three structural genes, Z, Y, and A, which encode enzymes that are required for the cell to use lactose as an energy source.
There is also a leader sequence just upstream of the structural genes.
The regulatory region of the lac operon includes a promoter, which serves as a binding site for RNA polymerase.
And then there is the operator, a regulatory sequence that is critical to understanding how the lac operon is regulated.
Next, we move on to the lac I gene, which is located upstream of the lac operon.
It is not directly adjacent, but it is shown here as such for simplicity.
Lac I encodes a key regulator of the lac operon known quite simply as the repressor protein.
The repressor has two different binding sites that we must consider.
First, there is an operator-binding site that enables binding to the operator.
Next, there is a lactose-binding site which binds to the sugar lactose if it is present.
As you will soon see, both binding sites play important roles in the regulation of the lac operon.
Finally, lactose itself is an important regulator. The absence or presence of lactose determines whether the lac operon is repressed, turned off, or induced, turned on.
Now let's consider the conditions under which the lac operon is repressed.
It would be a waste of energy for the cell to produce enzymes needed for lactose metabolism when lactose is not available.
Thus, when lactose is not present, the lac operon is repressed.
This makes sense, right?
But let's see how it works Despite the fact that RNA polymerase can bind to the promoter, the repressor protein binds to the operator and blocks transcription.
It physically gets in the way. Thus the structural genes are not transcribed.
Now let's explore the conditions under which the lac operon is induced, or turned on. When lactose is present, the lac operon is induced. This makes sense, because the cell cannot take advantage of lactose as an energy source until it produces the proteins needed for lactose metabolism.
But how does this work?
Lactose binds to the repressor, which causes an allosteric change in the operator-binding site and the repressor cannot bind to the operator.
Unimpeded, RNA polymerase transcribes the structural genes which are then translated into the enzymes needed for utilizing lactose as an energy source.
OK. That's it. I think you have it, but I want to leave you with a final thought.
The concepts that underlie the regulation of the lac operon also apply to many other examples of gene regulation in bacteria.
So you are now ready to apply your knowledge to the analysis of other regulatory systems.
Thank you for watching.