Uridine monophosphate (UMP) kinase is a conserved enzyme that catalyzes the ATP-driven conversion of uridylate monophosphate into uridylate diphosphate, an essential metabolic step. In prokaryotes, the enzyme exists as a homohexamer that is regulated by various metabolites. Whereas the enzymatic mechanism of UMP kinase (UK) is well-characterized, the molecular basis of its regulation remains poorly understood. Here we report the crystal structure of UK from Bacillus anthracis (BA1797) in complex with ATP at 2.82 A resolution. It reveals that the cofactor, in addition to binding in the active sites, also interacts with separate binding pockets located near the center of the hexameric structure. The existence of such an allosteric binding site had been predicted by biochemical studies, but it was not identified in previous crystal structures of prokaryotic UKs. We show that this putative allosteric pocket is conserved across different bacterial species, suggesting that it is a feature common to bacterial UKs, and we present a structural model for the allosteric regulation of this enzyme.