Assembly of protein complexes is considered a post-translational process involving random collision of subunits. We show that within the Escherichia coli cytosol, bacterial luciferase subunits LuxA and LuxB assemble into complexes close to the site of subunit synthesis. Assembly efficiency decreases markedly if subunits are synthesized on separate mRNAs from genes integrated at distant chromosomal sites. Subunit assembly initiates cotranslationally on nascent LuxB in vivo. The ribosome-associated chaperone Trigger Factor delays the onset of cotranslational interactions until the LuxB dimer interface is fully exposed. Protein assembly thus is directly coupled to the translation process, and involves spatially confined, actively chaperoned cotranslational subunit interactions. Bacterial gene organization into operons therefore reflects a fundamental mechanism for spatiotemporal regulation vital to effective cotranslational protein complex assembly.