The CO₂/O₂ specificity of single-subunit ribulose-bisphosphate carboxylase from the dinoflagellate, Amphidinium carterae

Some dinoflagellates have been shown recently to be unique among eukaryotes in having a ribulose-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39) composed of only one type of subunit, the 53-kDa large subunit [reviewed by Palmer, J.D. (1996) Plant Cell 8, 343-345]. Formerly, such homomeric Rubiscos bad been found only in anaerobic bacteria and are characterised by such poor abilities to discriminate against the competitive alternate substrate, O₂, that they would not be able to support net carbon gain if exposed to the current atmospheric CO₂/O₂ ratio. The capacity of Rubiscos from aerobic organisms to discriminate more effectively against O₂ appeared to correlate with the presence of additional 12- to 18-kDa small subunits. Thus the CO₂/O₂ specificity of the homomeric dinoflagellate Rubisco is of considerable interest from the structural, physiological and evolutionary viewpoints. However, for unknown reasons, Rubiscos from dinoflagellates studied so far are so unstable after extraction from the cells that kinetic characterisation has not been possible. We redesigned two methods for measuring Rubisco's CO₂/O₂ specificity to adapt them to rapid measurement at 10°C using unfractionated cell extracts. Both methods revealed that the CO₂/O₂ specificity of Rubisco from the dinofiagellate, Amphidinium carterae Hulburt, was approximately twice as great as that of other homomeric Rubiscos but unlikely to be sufficient to support dinoflagellate photosynthesis without assistance from an inorganic-carbon-concentrating mechanism.