<p>Members of this group are eukaryotic bifunctional enzymes with glutamine amidotransferase (<db_xref db="INTERPRO" dbkey="IPR000991"/>) and cyclase activities (<db_xref db="INTERPRO" dbkey="IPR006062"/>) that catalyse the fifth and sixth steps of the histidine biosynthetic pathway. In eubacteria, these steps are catalysed by a complex formed by two subunits, namely the glutamine amidotransferase HisH (<db_xref db="INTERPRO" dbkey="IPR016226"/>) and the cyclase HisF (<db_xref db="PIRSF" dbkey="PIRSF001454"/>) which are encoded in the same operon [<cite idref="PUB00027820"/>].</p> <p>The catalytic activity has been assessed for many members, mainly by complementation assays. The <taxon tax_id="4932">Saccharomyces cerevisiae</taxon> (Baker s yeast) member is able to suppress His auxotrophy of corresponding <taxon tax_id="562">Escherichia coli</taxon> hisH and hisF mutants [<cite idref="PUB00017574"/>]. In plants, hisHF cDNA from <taxon tax_id="162425">Emericella nidulans</taxon> (Aspergillus nidulans) and <taxon tax_id="3702">Arabidopsis thaliana</taxon> (Mouse-ear cress) complemented a S. cerevisiae <i>his7Delta</i> strain [<cite idref="PUB00017573"/>, <cite idref="PUB00027875"/>]. In addition, A. nidulans cDNA complemented E. coli <i>hisH</i> and <i>hisF</i> mutant strains [<cite idref="PUB00027875"/>].</p>