Most prokaryotic arginine deiminase pathways are under the control of a repressor gene, termed ArgR. This is a negative regulator, and will only release the arginine deiminase operon for expression in the presence of arginine. The crystal structure of apo-ArgR from Bacillus stearothermophilus has been determined to 2.5A by means of X-ray crystallography. 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 paralogoussequences show long branches and differ significantly from the well-conserved C-terminal region.
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