The genes for the large and small subunits of ribulose bisphosphate carboxylase/oxygenase (Rubisco) from Anacystis nidulans have been expressed in Escherichia coli under the control of the lac promoter to produce active enzyme. The enzyme can be purified from the cells to yield up to 200 mg Rubisco/l cultured bacteria, and is indistinguishable from the enzyme extracted from A. nidulans. In order to investigate the role of the N-terminus of the large subunit in catalysis, chimaeric genes were constructed where the DNA coding for the 12 N-terminal amino acids in A. nidulans was replaced by DNA encoding the equivalent, but poorly conserved, region of either the wheat or maize large subunit. These genes, in constructs also containing the gene for the A. nidulans small subunit, were expressed in E. coli and produced enzymes with similar catalytic properties to the wild-type Rubisco of A. nidulans. In contrast, when the N-terminal region of the large subunit was replaced by unrelated amino acids encoded by the pUC8 polylinker, enzyme activity of the expressed protein was reduced by 90% under standard assay conditions, due to an approximately tenfold rise in the Km for ribulose 1,5-bisphosphate. This confirms that the N-terminus of the large subunit has a function in catalysis, either directly in substrate binding or in maintaining the integrity of the active site.