In addition to repression and induction, some operons are only transcribed if a specific activator protein is bound to the DNA, and these are said to be under positive control for transcription. In these cases, even with the assistance from the correct sigma factor, RNA polymerase when it binds to the promoter does so weakly. Activator proteins therefore help the RNA polymerase recognize the promoter and bind more tightly.
Now, the activator binding site may be relatively close to the promoter, as we see in this first example, or it may be several hundred base pairs away from the promoter. And if it's several hundred base pairs away, then the activator protein will promote the DNA looping that we see here to make the necessary contacts for transcription. to proceed. In some cases, these activator proteins need additional binding of other molecules before they can bind to the activator binding site.
So our example here is maltose. Maltose operon, this is in E. coli, and it will regulate the transcription of enzymes necessary for the catabolism of maltose. Maltose is another energy source that E.
coli can use. In this circumstance, in this slide, Maltose is not present. So we don't actually need to transcribe these genes. And so the activator protein, the specific one for the Maltose operon is called MALT.
It cannot bind to the activator binding site. It doesn't have the right shape. And what that means is this RNA polymerase can't really bind either. So it might bind weakly. It might fall off very frequently.
It might not bind at all. and transcription therefore does not proceed. However, if maltose is present, maltose can now act as the effector molecule or the inducer molecule.
That means it can bind to multi-activator proteins and change the conformation of those molecules. They are now able to bind to the activator binding site. That means that RNA polymerase can now bind to the promoter and transcription can proceed.
Now this multi-activator protein can bind to other activator binding sites and these are collectively when something can happen where it can bind to other activated binding sites and controls transcription of more than one operon, these operons are collectively known as regulons. So regulons is a group of operons under the control of a single regulatory protein.