There are two main mechanisms for the formation of pseudoUridine in RNA. The one only depends on the pseudoUridine synthase, in which a protein plays both a recognition and a catalytic role, that is RNA-independent pseudouridylation. And the other relies on a complex formed by a class of H/ACA box small nucleolar RNA (snRNA) and a corresponding protein, in which the RNA plays a recognition role while the protein bound to RNA acts as a catalyst, that is RNA-dependent pseudouridylation. In prokaryotes, only the first mechanism exists. In eukaryotes, both mechanisms are present.