<p>Pseudouridine synthases catalyse the isomerisation of uridine to pseudouridine (Psi) in a variety of RNA molecules, and may function as RNA chaperones. Pseudouridine is the most abundant modified nucleotide found in all cellular RNAs. There are four distinct families of pseudouridine synthases that share no global sequence similarity, but which do share the same fold of their catalytic domain(s) and uracil-binding site and are descended from a common molecular ancestor. The catalytic domain consists of two subdomains, each of which has an alpha+beta structure that has some similarity to the ferredoxin-like fold (note: some pseudouridine synthases contain additional domains). The active site is the most conserved structural region of the superfamily and is located between the two homologous domains. These families are [<cite idref="PUB00045922"/>]:</p><p> <ul> <li>Pseudouridine synthase I, TruA.</li><li>Pseudouridine synthase II, TruB, which contains and additional C-terminal PUA domain.</li><li>Pseudouridine synthase RsuA (ribosomal small subunit) and RluC/RluD (ribosomal large subunits), both of which contain an additional N-terminal alpha-L RNA-binding motif.</li><li> Pseudouridine synthase TruD, which has a natural circular permutation in the catalytic domain, as well as an insertion of a family-specific alpha+beta subdomain.</li> </ul> </p><p> This entry represents the RluC/RluD subfamily of pseudouridine synthases. RluC and RluD are homologous enzymes which each convert three specific uridine bases in <taxon tax_id="562">Escherichia coli</taxon> ribosomal 23S RNA to pseudouridine: bases uracil-955, U-2504, and U-2580 in the case of RluC and uracil-1911, U-1915, and U-1917 in the case of RluD [<cite idref="PUB00032035"/>]. RluD also possesses a second function related to proper assembly of the 50S ribosomal subunit that is independent of Psi-synthesis [<cite idref="PUB00037409"/>]. Both RluC and RluD have an N-terminal S4 RNA binding domain. Despite the conserved topology shared by RluC and RluD, the surface shape and charge distribution are very different. </p>