16. Regulation of Expression
The Trp Operon
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In this video, we're going to begin our lesson on the triptophan. And so tryptophan is an amino acid. And recall that amino acids are the monomers or the building blocks of proteins. And so tryptophan is an amino acid whose three letter code or three letter abbreviation is trp or TRP. And so tryptophan is an amino acid, and tryptophan itself can either be absorbed by the cell from the surrounding environment or taken in from the surrounding environment, or tryptophan can be synthesized from scratch by the cell. Now the trp operon is a repressible operon, which recall from our previous lesson videos means that the trip operon is usually on or active, but it can be turned off, meaning that it can be repressed, and that's why it's called a repressible operon. And so the tripeperon is a repressible operon with 5 genes, and these 5 genes are all related to each other because they encode enzymes that are required for synthesizing tryptophan from scratch. And so the trp operon is responsible for, encoding enzymes required for synthesizing tryptophan from scratch. Now, if we take a look at our image down below, notice that we're showing you an image of the TRP operon, which has these five genes, TRP, e, d, c, b, and a. And, these five genes are important for encoding enzymes for synthesizing tryptophan from scratch. And so, of course, the trp operon has a TRP operator where the regulatory protein will bind, and it also has a TRP promoter as well where the RNA polymerase will bind. Now, taking a look over on the left hand side, notice that there is a TRP regulatory gene, which is TRP r. And so TRP r is the regulatory gene that is going to encode the TRP repressor protein. And the Trp repressor protein is going to be a repressor, and so that means that it will bind to the operator and inactivate transcription. It will repress transcription. However, the TRP repressor protein is actually expressed in the inactive form, which means that when it is initially expressed, it is inactive and it will not bind to the operator. The inactive TRP r repressor protein requires a corepressor in order for it to become active and bind to the operator to repress transcription. And so the corepressor for the inactive trp r is usually going to be tryptophan itself. And so, when tryptophan, this corepressor, is bound to the inactive TRP r, it activates TRP r, and allows TRP r to bind to the operator. And when it's bound to the operator, it can repress transcription. And so if we take a look at our image down below, we can get a better understanding of this here. And so the tripperon, again, contains 5 genes required for synthesizing tryptophan, for tryptophan synthesis, and it is regulated by the TRP repressor. And so over here we have our TRP regulatory gene where you can see the TRP repressor gene, the, TRP r gene, and it has its own promoter here. So it will be transcribed and translated into the repressor protein. However, when the TRP r is expressed, it's expressed in the inactive form. And so notice we have the inactive Prp repressor, and the inactive Prp repressor on its own, it cannot bind to the operator. 1st, the inactive trip repressor needs to be bound by a co repressor. And the co repressor is usually tryptophan itself, so tryptophan can bind to the inactive trippressor to form an active trippressor. And the active trippressor with tryptophan bound can bind to the operator and repress and block transcription of the tripperon genes. And so basically, what we're seeing here is that, the trp operon, is going to be repressed in the presence of tryptophan. When tryptophan is available and present, it can serve as a corepressor to activate the trippressor, and that will block transcription. And so this here concludes our brief introduction to the trp operon. But as we move forward in our course, we're going to take a closer look at the trp operon in the presence and in the absence of tryptophan itself. And so I'll see you all in our next video.
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