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 had 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 dinoflagellate, 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.