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ENVIRONMENTAL STRESS AND ADAPTATION

An Anaplerotic Role for Mitochondrial Carbonic Anhydrase in Chlamydomonas reinhardtii¹

Dipartimento di Scienze del Mare, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy (M.G., A.N.); Umeå Plant Science Center, Umeå University, S901 87 Umeå, Sweden (M.F., M.E.); and Division of Environmental and Applied Biology, School of Life Sciences, University of Dundee, Dundee DD1 4HN, United Kingdom (J.A.R.)

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 CO₂ 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.

² These authors contributed equally to this paper.

^* Corresponding author; e-mail giordano{at}unian.it; fax 39–071–220–4650.

Received March 20, 2003; returned for revision April 24, 2003; accepted May 6, 2003.

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