Pseudouridine synthases catalyze the site-specific isomerization of uridine residues that are already part of an RNA chain, which is the most abundant post-transcriptional modification of cellular RNAs. Their activity is specific for a particular RNA (e.g. tRNA or rRNA class). They can be classified into five families named after the E. coli enzymes RluA, RsuA, TruA, TruB, and TruD. (Adapted from PMID:17113994.)