This group is comprised of bacterial proteins similar to Escherichia coli RluF. Pseudouridine synthases catalyze the isomerization of specific uridines in an RNA molecule to pseudouridines (5-ribosyluracil, psi). No cofactors are required. E.coli RluF makes psi2604 in 23S RNA. psi2604 has only been detected in E. coli. It is absent from other eubacteria despite a precursor U at that site and from eukarya and archea which lack a precursor U at that site.