Many photosynthetic organisms employ a CO₂concentrating mechanism (CCM) to increase the rate of CO₂fixation via the Calvin cycle. CCMs catalyze ≈50% of global photosynthesis, yet it remains unclear which genes and proteins are required to produce this complex adaptation. We describe the construction of a functional CCM in a non-native host, achieved by expressing genes from an autotrophic bacterium in anEscherichia colistrain engineered to depend on rubisco carboxylation for growth. Expression of 20 CCM genes enabledE. colito grow by fixing CO₂from ambient air into biomass, with growth in ambient air depending on the components of the CCM. Bacterial CCMs are therefore genetically compact and readily transplanted, rationalizing their presence in diverse bacteria. Reconstitution enabled genetic experiments refining our understanding of the CCM, thereby laying the groundwork for deeper study and engineering of the cell biology supporting CO₂assimilation in diverse organisms.