design?

Truman A dataset based on all known functional sequences of ubiquitin (UB) has been prepared. This confirmed the extraordinary intolerance of this protein to substitution of alternative amino acids. Probabilistic calculations suggest evolutionary processes did not produce such unusual sequences. It was discovered that three amino acid positions on the chain are distinctive for organisms belonging to the animal, plant and fungus kingdoms. A fourth variant has already been reported. The unexpected identification of three new distinct classes of UB has implications for evolutionary theory. It requires the common ancestor for all UB families to be essentially simultaneous with the origin of the first eukaryote