The microorganism Methanosarcina acetivorans lives off everything it can metabolise into methane. How it finds its sources of energy, is not yet clear. Scientists at the Ruhr-Universität Bochum together with colleagues from Dresden, Frankfurt, Muelheim and the USA have identified a protein that might act as a "food sensor." They characterised the molecule in detail and found both similarities and differences to the system that is responsible for the search for food in bacteria.
MsmS has a different function to that thought
The protein MsmS has so far only been studied from a bioinformatics point of view. Computer analyses of its gene sequence had predicted that it might be a phytochrome, i.e. a red light sensor. Using spectroscopic methods, the research team of the current study have refuted this theory. MsmS has a heme cofactor, like haemoglobin in red blood cells, and can, among other things, bind the substance dimethyl sulphide. This is one of the energy sources of Methanosarcina acetivorans. MsmS might thus serve the microorganism as a sensor to directly or indirectly detect this energy source. In genetic studies, the scientists also found evidence that MsmS regulates systems which are important for the exploitation of dimethyl sulphide.
Archaea: flexible "eaters"
Methanosarcina acetivorans belongs to the Archaea which constitute the third domain of life, alongside Bacteria and Eukarya; the term Eukarya comprising all living organisms with a cell nucleus. Many of them are adapted to extreme conditions or are able to use unusual energy sources. Among the organisms that live from methane production, the so-called methanogenic organisms, M. acetivorans is one of the most flexible when it comes to the choice of food sources. It converts many different molecules into methane, and thus produces energy. How M. acetivorans detects the different food sources, is still largely unknown.
In Archaea, unlike bacteria
For this purpose, bacteria use the so-called two-component system: when a sensor protein comes in contact with the food source, the protein modifies itself; it attaches a phosphate group to a certain amino acid residue, the histidine. The phosphate group is then transferred to a second protein. In methanogenic organisms such a process could trigger cellular processes that activate the methane production. Archaea might also use comparable sensor proteins in a way similar to bacteria. MsmS would be a candidate for such a task, because the analyses of the research team showed that it is able to transfer a phosphate residue to an amino acid. The target site of this phosphorylation is, however, probably not histidine. "So there could be differences between the signal transduction systems of Archaea and Bacteria" speculates Prof. Dr. Nicole Frankenberg-Dinkel from the work group Physiology of Microorganisms. "It is also interesting that the heme cofactor is covalently bound, i.e. linked with the protein by an electron-pair bond. This is very uncommon for sensor proteins which are present in the cell fluid."
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