IM BACK! Let’s Learn About Operons
Hello chat! I have returned. Let’s talk about operons.
The purpose of operons is to group genes together under one promoter, which allows for coordinated gene expression. This is especially helpful when multiple genes are involved in producing multiple proteins which interact with each other stepwise to create a product. Because this kind of process requires molecular coordination, it makes sense that we would want an organized system to regulate these behaviors.
The great thing about operons and their use of the same promoter is that this system can be controlled by environmental factors. The environment just needs to exert some effect on the promoter, and the whole chain reaction will occur for the cell to respond appropriately.
When studying operons, there are two common examples used to explain this concept. Before I introduce these examples, I’d like to define two terms:
An inducible operon can be induced by the environment. This means that, in the absence of any stimulus, the operon is in the “off” state, and the genes are inactive. No protein is produced.
A repressible operon can be repressed by the environment. This means that, in the absence of any stimulus, the operon is in the “on” state, and the genes are active. Protein is produced.
In this article, I will be discussing two operons: the lac operon and the trp operon from E. coli. The lac operon is inducible and the trp operon is repressible.
Each operon has the following key points:
sequence of genes / other factors involved
gross regulation mechanism
fine regulation mechanism
explanation of how the elements work together to function
These points will organize each section below so it’s easier to understand.
The lac operon - catabolic, inducible
The lac operon contains the following sequence of genetic material: promoter for the lac inhibitor gene, lac inhibitor gene, CAP site, promoter, operator, lacZ gene, lacY gene, and lacA gene. The purpose of the operon is to produce enzymes required for lactose digestion and associated metabolism.
The gross regulation mechanism is the presence of allolactose, while the fine regulation mechanism depends on cAMP/glucose.
Here’s how it works:
When the environment contains lactose, it will naturally also contain a small amount of a lactose isomer called allolactose. Allolactose inhibits the protein produced by the lac inhibitor (lacI) gene, which means that it actually activates the lac operon due to inhibiting the inhibitor. Thus, allolactose is an inducer. In summary: lactose present means genes on, lactose absent means genes off.
Glucose levels in the environment are also related to the activity of the lac operon. Glucose being present means that the organism is actively conducting cellular respiration to metabolize this energy. cAMP, a molecule used in cellular respiration, will thus not be available as it is preoccupied by the ongoing respiration. Glucose being absent thus means that cAMP is abundant. In the absence of glucose, cAMP will bind to the CAP site, and the cAMP-CAP complex will bind the DNA and induce transcription by promoting RNA pol’s binding to the promoter. In summary: glucose present means genes off, glucose absent means genes on.
Let’s think about the context of these functions. Lactose and glucose are energy sources. Glucose is a more directly beneficial energy source, so it’s the primary preference. If glucose is present, the lactose-digesting apparatus is not necessary. But if the organism needs energy and glucose is not available, it will settle on lactose. If both are not present, the operon does not need to be active; the organism will just starve because there’s nothing to digest at the moment.
Here’s a summary image:
lacZ - produces beta galactosidase/lactase to break down the lactose.
lacY - produces lactose permease (membrane protein) which allows lactose to be moved into cells for processing.
lacA - produces transacetylase which adds acetyl groups on lactose, allowing for metabolism.
The trp operon - anabolic, repressible
The trp operon contains the following sequence of genetic material: trp repressor gene, promoter, operator, leader sequence, trpE gene, trpD gene, trpC gene, trpB gene, and trpA gene. The purpose of the operon is to produce tryptophan.
The gross regulation mechanism is the presence of trp, while the fine regulation mechanism depends on stem loops.
Here’s how it works:
Tryptophan from the environment activates the trpR (trp repressor) gene and promotes expression of trpR, thereby inhibiting expression of the actual genes on the operon. Thus, trp from the environment is a corepressor. A key point about this process is that it blocks RNA pol recruitment, which means that the initiation of transcription is blocked.
This is in contrast with the attenuation mechanism of the trp operon, which blocks the completion of transcription instead of blocking the initiation. Between the operator and trp synthesis genes lies a palindromic leader sequence. If the environment has a lot of tryptophan, we will not have to wait a long time for a trp-attached tRNA to come help build the amino acid (E. coli are prokaryotic, so transcription and translation can occur at the same time since the processes aren’t membrane-bound). Thus, we can get through most of the leader sequence, and the later part of the leader becomes a stem loop (3-4 loop). However, when we get stalled waiting for a trp-loaded tRNA, the stem loop forms earlier (2-3 loop). The 3-4 loop is the terminator because it’s large and doesn’t allow the ribosome through, and it halts trp synthesis. 2-3 is anti-terminator (small enough to allow the ribosome through) and encourages completion of transcription.
Here’s a summary image:
trpE-A are just trp biosynthesis genes.
So yeah here’s some resources:
