Allosteric enzymes are part of a unique class of enzymes which regulate metabolic pathways. On the molecular level, allosteric regulation is the result of interactions between discrete binding sites on the enzyme. In order to accommodate these multiple binding sites, allosteric enzymes have evolved with oligomeric quaternary structures. However, only a few oligomeric enzymes are known to have regulatory interactions between binding sites. Is regulatory activity an inherent property of oligomeric enzymes? The trimeric Bacillus subtilis aspartate transcarbamoylase catalyzes the first committed step of the pyrimidine biosynthetic pathway and is not known to be a regulatory enzyme. When an alanine residue is substituted for the active-site residue Arg-99 by site-specific mutagenesis, the regulatory activity of homotropic substrate cooperativity (Hill coefficient of 1.5) is observed in the resulting mutant enzyme. These results suggest that homotropic regulation may have evolved by a relatively small number of mutations to an oligomeric enzyme.