two of the most studied operons are the trip operon and the Lac operon and what I want to do in this video is focus on the trip operon which is essential for the production of tryptophan trip tryptophan which you might recognize as an amino acid often associated with Thanksgiving and turkey dinner but tryptophan as as all or most amino acids are essential for creating the polypeptides the proteins that you use in your body and so the trip operon and here we're going to be talking about not your body well we're going to be talking about something that's in your body we're going to talk about eoli it is an operon that is on the eoli that is part of the eoli genome and just in this diagram the way it's drawn it would be sitting it would be sitting right over here and just as a reminder an operon is a combination of a set of genes as well as the regulatory DNA sequences for that set of genes in particular you have the promoter you have the operator right over here the promoters were the RNA polymerase binds and would start the transcription process The Operators were repressor binds and this is going to be essential for understanding how the trip operon works and so what do these genes actually code for well these genes code enzymes that are used in the construction of tryptophan and I'm always am amazed that enzymes can be used to construct what what what are essentially molecules that are much smaller than the enzymes themselves in fact the enzymes involve are made up of amino acids but then they're used to make particular amino acids and so trip e d c ba a they're all once they are once they are transcribed into mRNA and then translated in the ribosomes these enzymes are used to create tryptophan for tryptophan biosynthesis so let's think about how this works so if we are in a low tryptophan environment our eoli it needs tryptophan it needs that amino acid as a building block for its proteins so in that world it makes sense that in a low tryptophan environment the RNA polymerase can just latch on to the promoter and begin the transcription process transcribe these these five genes in in into RNA which then can be translated into the into those enzymes and then you will have more tryptophan biosynthesis that makes sense that you want to create tryptophan if you're in an environment that does not have a lot of tryptophan but what if we did have a lot of tryptophan well if you have a lot of something around you shouldn't waste energy creating more of it you have to appreciate that all organisms that are around today are the byproducts of billions of years of Evolution and they've learned to be very careful or or the ones that are selected for tend to be the ones that don't waste resources and so when you have tryptophan around you probably don't want this transcription to occur so it would make sense that maybe tryptophan can act as a co-repressor for a repressor molecule for a repressor enzyme that would attach to the operator and block the RNA polymerase from transcribing and that's exactly what happens so when you're in a tryptophan environment and tryptophan obviously does not look like these little yellow quadrilaterals over there but that's just for our visualization purposes and neither does RNA polymerase look like that or neither does the the trip the trip recept repressor look like that in fact I encourage you to web searches and see how they actually look they're they're fascinating but when you have a lot of tryptophan the tryptophan can act as a co- co- repressor it can bind to the trip repressor essentially activate it so that it it'll change its confirmation so that it can then attach to the operator in the operon and once it's attached to the operator well then the RNA polymerase can no longer move forward with transcription so as you can see this is a very valuable feedback loop or not even necessarily feedback if you're in an environment with a lot of tryptophan don't create tryptophan or if you just have a lot of tryptophan laying around don't create more tryptophan if you don't have tryptophan around well then the repressor won't be co- repressed I guess you can say and then the tryptophan will actually be created now tryptophan is an interesting thing because the control of transcription isn't the only place where you have some type of a a feedback loop or or kind of a conditional situation you can actually have direct feedback inhibition between the proteins and so this part isn't related to the transcription but if this is a precursor of tryptophan it's all very abstract in this diagram and let's say enzyme One turns into precursor 2 enzyme 2 turns into precursor 3 and enzyme 3 turns it into tryptophan well you actually have direct feedback inhibition where tryptophan can then bind or interact with enzyme one here could interact with enzyme one let me do it in a color you could see could interact with enzyme one so that it can no longer act as efficiently taking precursor one to precursor 2 so this right over here this is a this is the classic feedback in ition [Music] feedback inhibition the focus of this video we're talking about operons and Gene regulation but it's important to realize that the that the regulation of of the creation of tryptophan doesn't only occur at the transcription level and I'm not going to go into this video it's a slightly more advanced topic but there's also uh uh regulation of tryptophan biosynthesis through a process called attenuation which doesn't affect the start of transcription but it affects uh how things get completed and it will keep tryptophan from being completely or or the the the entire process from from going to completion but the ones that are are most typically talked about are what we just talked about here where you have your tryptophan X as a co- repressor of the trip repressor and also the feedback inhibition which once again is not really about Gene regulation but you can see how the product of this process can go back and inhibit one of the first enzymes