Metabolic Intricacies: It's More Than Just Genes and Proteins…
Transcription and translation are controlled so that just the right amount of protein product is produced.
That is, the cell “knows” when to turn on a gene to express a particular protein and when to turn off protein production.
This is done through the workings of operons.
Two Ways to Regulate Metabolism
Regulation of gene expression: the operon
When the end product of a metabolic pathway is in great enough concentration, it will repress expression of the genes for all the enzymes needed for the pathway.
E. coli is able to synthesize tryptophan (one of the twenty amino acids) in a series of five steps.
Each step is catalyzed by an enzyme.
Each enzyme is encoded by a different gene.
The five genes that code for these five enzymes are located next to each other on the same stretch of DNA.
Therefore, all five genes are served by only one promoter.
A promoter is a region on the DNA where RNA polymerase binds so that transcription can begin.
This grouping under one promoter allows all of the genes to be expressed or not expressed together.
The genes are functionally related: when an E. coli needs to make tryptophan, it needs to make all five enzymes.
Clustering the genes together under one promoter also allows them to be switched on and off as a group.
The switch that controls access of RNA polymerase to the genes is known as the operator.
It is located between the promoter and the enzyme-coding genes.
By putting all five genes under the control of one switch, the system is more efficient.
The Operon
The entire mechanism controlling the expression of these genes is known as the trp operon.
All operons are composed of three parts:
The promoter: the place where RNA polymerase binds
The operator: an “on-off switch” to control transcription of the enzyme-coding genes
The genes: genes that code for enzymes involved in a metabolic pathway
There are many different kinds of operons—whenever there is a series of genes that produce enzymes in a metabolic pathway, there is the possibility they may be part of an operon.
There are two reasons why the repressor protein doesn't permanently switch off the operon, despite its continual production and diffusion:
The binding of the repressor protein to the operator is reversible and depends on the repressor concentration.
The repressor protein is an allosteric enzyme, and therefore has an active and an inactive shape.
The repressor protein is synthesized in its inactive form.
The repressor protein only changes to its active form when the final product (tryptophan for the trp operon) binds to its allosteric site.
Tryptophan is therefore a corepressor—a small molecule that cooperates with a repressor protein to switch an operon off.
When the level of tryptophan increases, it binds to the repressor protein, causing it to take its active form.
The repressor, now active, binds to the operator, preventing transcription.
The production of tryptophan is thus curtailed.
The trp operon is called a repressible operon because it is by default switched on, and therefore the only way to control it is to switch it off—i.e., to repress it.
