An anaplerotic role for mitochondrial carbonic anhydrase in Chlamydomonas reinhardtii

Previous studies of the mitochondrial carbonic anhydrase (mtCA) of Chlamydomonas reinhardtii showed that expression of the two genes encoding this enzyme activity required photosynthetically active radiation and a low, CO, concentration. These studies suggested that the mtCA was involved in the inorganic carbon-concentrating mechanism. We have now shown that the expression of the mtCA at low CO2 concentrations decreases when the external NH4+ concentration decreases, to the point of being undetectable when NH4+ supply restricts the rate of photoautotrophic growth. The expression of mtCA can also be induced at supra-atmospheric partial pressure of CO2 by increasing the NH4+ concentration in the growth medium. Conditions that favor mtCA expression usually also stimulate anaplerosis. We therefore propose that the mtCA is involved in supplying HCO3- for anaplerotic assimilation catalyzed by phosphoenolpyruvate carboxylase, which provides C skeletons for N assimilation under some circumstances.