A 36 Kilodalton Limiting-CO2 Induced Polypeptide of Chlamydomonas Is Distinct from the 37 Kilodalton Periplasmic Carbonic Anhydrase

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Abstract
Chlamydomonas reinhardtii possesses a CO2-concentrating mechanism, induced by limiting CO2, which involves active transport and accumulation of inorganic carbon within the cell. Synthesis of several proteins is induced by limiting CO2, but of those, only periplasmic carbonic anhydrase has an identified function in the system. No proteins involved in active transport have yet been identified, but induced, membrane-associated polypeptides, such as the 36 kilodalton polypeptide focused on in this paper, would seem to be candidates for such involvement. The 36 kilodalton polypeptide was shown to be synthesized de novo upon transfer of cells limiting CO2. It was purified using SDS-PAGE and used to produce polyclonal antibodies. Antibodies were used to confirm the air-specific nature of the polypeptide, its strict association with membrane fractions, and the time course of its induction. Using the antibodies, a single, 36 kilodalton polypeptide was found to be specifically immunoprecipitated from in vitro translation products of poly(A+) RNA from cells only after exposure to limiting CO2. The absence of translatable mRNA for this polypeptide in CO2-enriched cells indicated that regulation occurs at the level of message abundance. The antibodies were also used to demonstrate the distinction between the limiting-CO2 induced 36 kilodalton polypeptide and the similarly sized, limiting-CO2 induced periplasmic carbonic anhydrase.