Pseudouridine synthases (psi synthases) are a class of RNA modifying enzymes that catalyze the conversion of uridine to psi in a variety of RNA substrates, without any cofactor. This process is key for translational accuracy and efficiency. psi synthases, based on sequence homology, are grouped into six families all of which share catalytic domains with a common fold containing conserved motifs required for catalytic activity PMID:23707380. Structural domains comprising this superfamily share the structure of the the N-terminal region of the catalytic domain of Pseudouridine synthase I (TruA) which specifically modifies uridines at positions 38, 39 and/or 40 of tRNAs PMID:17466622.