Regulatory circuit of how bacteria sense environmental signals. Membrane bound anti-sigma factor releases a sigma factor into the cytosol after signal binding which modifies gene expression

How bacteria sense the presence of iron in their environment

While the last article was about the project I studied in the past, this one will be about the project I am currently working on. Generally, it is about how bacteria sense the presence of iron in an environment where there is only little iron present.

Okay, let’s start from the beginning. 

One reason why I am so fascinated with bacteria, is because they can basically survive in all kinds of different environments. However, these changing environments require proper preparations. Just as you would put on some layers of warm clothes in winter when you go outside, they need to adapt to the different conditions when they come from the cold and salty sea and end up in your warm body. 

Plus, the bacteria I am working with have two cellular membranes – the so called outer and inner membrane. This makes it even more tricky for a bacterium to know what is going on on the outside, because the information about the outside has to cross two barriers. 

Now, I look into how bacteria adapt to the low-iron environment for example in our body. Iron is an essential metal for all living organisms because it is needed for cellular respiration which is the key process to keep a cell alive. So bacteria need it and they are always keen to find some so they can flourish.

However, iron is not just floating around everywhere, because free iron is actually highly toxic for cells. Hence, all living organisms keep iron bound to another compound, which is like a specific iron vehicle. What is more, each organism has its own specific iron vehicle, for example the haemoglobin of our blood is one of such. 

So all the iron is bound and kept from bacteria, which makes it quite difficult for them to find any iron within our body. This is also one of our body’s immune defence mechanism to limit bacterial growth. But how do bacteria circumvent this and know that there is iron available and how do they catch it?

You can now follow this whole circuit in the little sketch I made. I will not bother with scientific names as I would rather have the general concept understood. Hence, I will just call each component as what they are depicted in the sketch.

Regulatory circuit of how bacteria sense environmental signals. Membrane bound anti-sigma factor releases a sigma factor into the cytosol after signal binding which modifies gene expression

Bacteria produce their own highly specific iron vehicles that catch iron (the brown vehicle with the red Fe on the outside) from the deepest corners of our body and bring the iron back to the bacterium. The iron loaded vehicle is transported into the cell via a specific transporter (the grey barrel) and this transport process is also used to inform the cell that there is iron on the outside.

So when an iron loaded vehicle binds to the transporter on the outside of the cell, the transporter activates its little signalling domain (the red bean) on the other side of the membrane. This red bean is like a messenger that hangs around between the two cell membranes and waits to receive the signal to go and find its counterpart – the pink bean – on the inner membrane. 

This pink bean is bound to the yellow pacman that sits on the other side of the inner membrane. The yellow pacman is actually an inhibitor and is called anti sigma factor. It has this name because it binds and keeps hold of the sigma factor, the green worm thingy.

So now the pink bean gets the signal from the red bean that there is iron available and it activates the dismounting of the yellow pacman. Dismounting the anti sigma factor/yellow pacman means that the sigma factor is set free and can go into the cell and bind its target machinery (in blue). 

The blue machine is the cell factory and is responsible to make proteins out of DNA by initiating the first step of protein production. The sigma factor is essential in this whole process as it helps the blue machine to find specific genes that are required at a specific moment. Hence, the sigma factor here shows the blue machine the gene for the iron transporter (amongst others).

The blue machine then produces many iron transporters that are sent to the outer membrane and transport more iron loaded vehicles into the cell.

Headache? Yeah me too… 🙂

Almost there.

Using such a mechanism, a bacterium makes sure it only produces these iron transporters when they are needed as they cost a lot of energy to produce. And the cell also assures that it captures as much iron as possible when it is actually available. 

Okay, this is just the basics of this regulatory mechanism, more on this can be read here. Depending on other metals or nutrients or bacteria, more components/beans/pacmans can be involved, which makes these mechanisms even more complicated but also super interesting to study. 

So this is how bacteria make sure that they know exactly what is going on in their surrounding. While we just have to look out of the window to see that it’s raining and we need an umbrella, they need a postman and a porter just to know that there is some iron on the other side.

Take away from this week’s article

  • to know what is going on in their surrounding bacteria couple environment-sensing mechanisms with the transport of nutrients
  • many different players are involved in handing over the information to the cell inside
  • the crucial step is the release of a sigma factor that activates the production of the specific transporter

3 thoughts on “How bacteria sense the presence of iron in their environment

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