<p>This entry represents a structural domain consisting of segregated alpha and beta regions in 3-layers. Homologous domains with this structure are found in:</p><p> <ul> <li>3,4-dihydroxy-2-butanone 4-phosphate synthase (<db_xref db="EC" dbkey="4.1.99.12"/>) (DHBP synthase) (RibB) </li><li>A family of eukaryotic and prokaryotic hypothetical proteins that includes YrdC and YciO from <taxon tax_id="562">Escherichia coli</taxon> and MTH1692 from the archaea <taxon tax_id="145262">Methanothermobacter thermautotrophicus</taxon> (Methanobacterium thermoformicicum)</li> </ul> </p><p> DHBP synthase RibB catalyses the conversion of D-ribulose 5-phosphate to formate and 3,4-dihydroxy-2-butanone 4-phosphate, the latter serving as the biosynthetic precursor for the xylene ring of riboflavin [<cite idref="PUB00003582"/>]. In <taxon tax_id="658">Photobacterium leiognathi</taxon>, the riboflavin synthesis genes ribB (DHBP synthase), ribE (riboflavin synthase), ribH (lumazone synthase) and ribA (GTP cyclohydrolase II) all reside in the lux operon [<cite idref="PUB00043824"/>]. RibB is sometimes found as a bifunctional enzyme with GTP cyclohydrolase II that catalyses the first committed step in the biosynthesis of riboflavin (<db_xref db="INTERPRO" dbkey="IPR000926"/>). No sequences with significant homology to DHBP synthase are found in the metazoa.</p><p>The YrdC family of hypothetical proteins are widely distributed in eukaryotes and prokaryotes and occur as: (i) independent proteins, (ii) with C-terminal extensions, and (iii) as domains in larger proteins, some of which are implicated in regulation [<cite idref="PUB00015683"/>]. YrdC from <taxon tax_id="562">Escherichia coli</taxon> preferentially binds to double-stranded RNA and DNA. YrdC is predicted to be an rRNA maturation factor, as deletions in its gene lead to immature ribosomal 30S subunits and, consequently, fewer translating ribosomes [<cite idref="PUB00043825"/>]. Therefore, YrdC may function by keeping an rRNA structure needed for proper processing of 16S rRNA, especially at lower temperatures. Sua5 is an example of a multi-domain protein that contains an N-terminal YrdC-like domain and a C-terminal Sua5 domain. Sua5 was identified in <taxon tax_id="4932">Saccharomyces cerevisiae</taxon> (Baker's yeast) as a suppressor of a translation initiation defect in the cytochrome c gene and is required for normal growth in yeast; however its exact function remains unknown [<cite idref="PUB00043826"/>]. HypF is involved in the synthesis of the active site of [NiFe]-hydrogenases [<cite idref="PUB00011084"/>].</p> DHBP synthase RibB-like alpha/beta domain