Arabinose Operon - Online Tutor, Practice Problems & Exam Prep

The arabinose operon regulates the breakdown of arabinose, a sugar, through transcriptional activation or repression. When arabinose is abundant, the ARAC protein binds to the araI site, aided by the cAMP-CAP complex, initiating transcription. In contrast, in the absence of arabinose, ARAC binds both araI and araO, creating a DNA loop that prevents polymerase binding, thus repressing transcription. This operon exemplifies catabolite repression, where energy is conserved by regulating gene expression based on substrate availability.

Arabinose Operon

So the arabinose operon is responsible for encoding genes that control the breakdown of arabinose, and what is that? It is a sugar. So, when there's a lot of arabinose present, the cell wants to break it down, and therefore, it does that through activating the operon. And if arabinose is not present, it doesn't want to waste the energy from producing those genes and those proteins. And so when arabinose is not there, it wants to repress the transcription of the operon. So how does it do that? Well, there are 2 regulatory regions you need to know about. The araI, which is also called the initiator, and the araO regions. And these are DNA sequences, actually, just upstream of the, arabinose operon. And so how are they activated or repressed? Well, there's a certain protein called ARAC. So when arabinose is present, so when there's an abundance of this sugar, the ARAC binds the araI side or the initiator side, and that initiates transcription, which makes sense. Right? But there's also a second thing that is also required for initiating transcription, and that is the cAMP cap complex. Now you may remember this complex, the glucose regulation of the lac operon. So if you're unfamiliar with how this complex works, or you've never heard of it before, I would go back and review this video because this, the glucose like operon video, really goes into detail how the camp cap complex works to activate operons. So, in this case, there's also a binding site for this complex, and that also helps to initiate transcription. So that's initiation. The ARAC binds the araI and the cAMP cap complex, but both are binding, and they activate transcription. But when arabinose is not present, how is that inactivated? How is that transcription repressed? Well, how it is is it's done through the same protein, ARAC, but instead of just binding the araI or the initiator site, it binds both the araI and the araO, and that represses transcription. Operon. So binding. So let me show you what that looks like. So here we have the operon. So, there would be genes here. Right? So I'm just drawing genes. There are, arabinose genes, part of this operon that would break down arabinose. So when we have the ARAC protein and when arabinose is present, what happens is it only binds the ARAA site. In addition to that, there's this camp cap complex that comes in and binds to an adjacent side as well, and this helps to activate the operon that will break down the sugar. Now, if arabinose is not present, so if I just scribble it out here, say, okay, well, it's not present, then what happens is the ARAC protein also binds the araO site, which is here. And when it does, it actually folds this DNA up to the side of the protein, and it will create a DNA loop. And when the DNA is looped, like this, then that prevents polymerases from binding, and therefore transcription cannot occur if the polymerase can't bind. So the ARAC protein, depending on which side it binds to, whether it's bound to just 1, the initiator, or both, that determines whether or not the arabinose operon is activated or inhibited. So with that, let's now move on.

The genes in the arabinose operon are responsible for what?

• A. Synthesizing arabinose
• B. Breaking down arabinose
• C. Converting arabinose into starch
• D. Synthesizing sugar

When arabinose is present, the arabinose operon is what?

• A. Active – meaning the genes are being transcribed
• B. Inactive – meaning the genes are not being transcribed
• C. Nothing, arabinose does not control the arabinose operon