<p>The arginine dihydrolase (AD) pathway is found in many prokaryotes and some primitive eukaryotes, an example of the latter being <taxon tax_id="5741">Giardia lamblia</taxon> (Giardia intestinalis) [<cite idref="PUB00008308"/>]. The three-enzyme anaerobic pathway breaks down L-arginine to form 1 mol of ATP, carbon dioxide and ammonia. In simpler bacteria, the first enzyme, arginine deiminase, can account for up to 10% of total cell protein [<cite idref="PUB00008308"/>].</p><p>Most prokaryotic arginine deiminase pathways are under the control of a repressor gene, termed ArgR [<cite idref="PUB00009717"/>]. This is a negative regulator, and will only release the arginine deiminase operon for expression in the presence of arginine [<cite idref="PUB00008310"/>]. The crystal structure of apo-ArgR from <taxon tax_id="1422">Bacillus stearothermophilus</taxon> has been determined to 2.5A by means of X-ray crystallography [<cite idref="PUB00009718"/>]. The protein exists as a hexamer of identical subunits, and is shown to have six DNA-binding domains, clustered around a central oligomeric core when bound to arginine. It predominantly interacts with A.T residues in ARG boxes. This hexameric protein binds DNA at its N terminus to repress arginine biosyntheis or activate arginine catabolism. Some species have several ArgR paralogs. In a neighbour-joining tree, some of these paralogous sequences show long branches and differ significantly from the well-conserved C-terminal region. </p>