How the animals lost their sensors
For free-living organisms, the ability to sense and respond to the outside environment is crucial for survival. Eukaryotes, such as animals and plants, often have highly complex network systems in place to monitor their surroundings and respond effectively, but bacteria have developed a remarkably simple system. It’s called the ‘Two Component System’ because it literally relies on just two components; a sensor and a responder. The sensor picks up the signal, communicates this to the responder, which then causes the effect.
The picture above shows this process happening. The ‘communication’ of the message from the sensor to the responder, as shown by the coloured arrows, is carried out by transferring phosphate molecules. The signal interacting with the sensor causes the sensor to autophosphorylate (phosphorylate itself) and then pass the phosphate molecule onto the responder to trigger the response. The letters “H” and “D” are the actual amino-acids being phosphorylated; Histadine and Aspartate.
Although Two-Component Systems (TCS) are found in all three superkingdoms of life (archaea, bacteria and eukaryotes) they are suspiciously absent from the animal kingdom. Plants have them, as do fungi and several protazoa, but they just aren’t present in animals. For this reason they’ve been looked into as potential antibiotic targets as knocking out the Two-Component Systems of most bacteria is fatal.
Why don’t animals use TCS? To answer this you have to start looking at the evolution of the system itself, because despite being nominally present in eukaryotes such as plants and fungi, TCS are used very differently. Bacteria use TCS for sensing a wide variety of signals; stress, metabolism, nutrient regulation, chemotaxis, pathogen-host interactions etc. In eukaryotes on the other hand they are used sparingly; for ethylene responses and photosensitivity in plants and osmoregulation in fungi and slime moulds.
