Acetyl-coA carboxylase (ACC) is a central metabolic enzyme that catalyzes the committed step in fatty acid biosynthesis: biotin-dependent conversion of acetyl-coA to malonyl-coA. The bacterial carboxyltransferase (CT) subunit of ACC is a target for the design of novel therapeutics that combat severe, hospital-acquired infections resistant to the established classes of frontline antimicrobials. Here, we present the structures of the bacterial CT subunits from two prevalent nosocomial pathogens, Staphylococcus aureus and Escherichia coli, at a resolution of 2.0 and 3.0 A, respectively. Both structures reveal a small, independent zinc-binding domain that lacks a complement in the primary sequence or structure of the eukaryotic homologue.