Was the RRM domain already present in the common universal ancestor?

Besides the light it sheds on the evolution of SR proteins, this gargantuan phylogenetic study also relates to the superstructure of the tree of life. By showing that a large number of bacteria present proteins containing a very similar RRM domain to that of eukaryotic SR proteins, it does indeed suggest that this domain potentially existed in the last universal common ancestor (LUCA). “This domain is extremely well conserved and presents identical motifs in eukaryotes and prokaryotes. Considering their complexity and phylogenetic proximity, it’s rather unlikely that these RRM domains appeared (or were transferred) independently in these two groups”, Denis Baurain explains.

While many scientists think that eukaryotes (far more complex, especially their machinery) result from the fusion between an archeon and a bacterium (hence, two prokaryotic organisms), others support the more traditional vision that archaea, bacteria and eukaryotes have evolved separately from LUCA. Even if the study of RRM doesn’t help us make a clear-cut decision between these two hypotheses, it does however invalidate the idea that LUCA was an organism with very simple machinery, which opens the way for the rehabilitation of the traditional hypothesis. “The RRM domain can be found in eukaryotes and some types of bacteria. We can therefore reasonably imagine that this domain was present in LUCA and that it then disappeared, during the evolution of archaea and some types of bacteria”, concludes Patrick Motte.

The importance of the phylogenetic study conducted by the two teams of the professors in Liège has drawn the attention of the scientific community. Hence, it not only featured on the cover of the Plant Physiology journal last February, but it also attracted the attention of Faculty of 1000, an international scientific organisation with more than 10 000 experts in biology and medicine who assess the quality of publications in these domains. It was very well received (“must read”) and recommended as an example for other phylogenetic studies to follow.