The chemical structures of cobamides [cobalamin (Cbl)-like compounds] are the same, except for the lower ligand, which in adenosylcobalamin (AdoCbl) is 5,6-dimethylbenzimidazole, and in adenosylpseudocobalamin (AdopseudoCbl) is adenine. Why the lower ligand of cobamides varies and what the mechanism of lower ligand replacement is are long-standing questions in the field of B 12 biosynthesis. Work reported here uncovers the strategy used by the photosynthetic α-proteobacterium Rhodobacter sphaeroides to procure the cobamide it needs to grow on acetate as a carbon and energy source. On the basis of genetic and biochemical evidence we conclude that, in R. sphaeroides, the activity of the cobyric acid-producing amidohydrolase CbiZ enzyme is essential for the conversion of AdopseudoCbl into AdoCbl, the cobamide needed for the catabolism of acetate. The CbiZ enzyme uses AdopseudoCbl as a substrate, but not AdoCbl. Implications of these findings for cobamide remodelling in R. sphaeroides and in other CbiZ-containing microorganisms are discussed. © 2009 Blackwell Publishing Ltd.