<p>This entry represents a family of FAD-dependent hydroxylases (monooxygenases) which are all believed to act in the aerobic ubiquinone biosynthesis pathway [<cite idref="PUB00013848"/>]. A separate set of hydroxylases, as yet undiscovered, are believed to be active under anaerobic conditions [<cite idref="PUB00013779"/>]. In <taxon tax_id="562">Escherichia coli</taxon>, three enzyme activities have been described: UbiB (which acts first at position 6, see <db_xref db="INTERPRO" dbkey="IPR010232"/>), UbiH (which acts at position 4, [<cite idref="PUB00013809"/>]) and UbiF (which acts at position 5, [<cite idref="PUB00013834"/>]). UbiH and UbiF are similar to one another and form the basis of this subfamily. Interestingly, E. coli contains another hydroxylase gene, called visC, that is highly similar to UbiF, adjacent to UbiH and, when mutated, results in a phenotype similar to that of UbiH (which has also been named visB) [<cite idref="PUB00013761"/>]. Several other species appear to have three homologs in this family, although they assort themselves differently on phylogenetic trees (e.g. Xylella and Mesorhizobium) making it difficult to ascribe a specific activity to each one. Eukaryotes appear to have only a single homologue in this subfamily (COQ6, [<cite idref="PUB00013831"/>]) which complements UbiH, but also possess a non-orthologous gene, COQ7 which complements UbiF.</p>