Low-CO 2 -inducible protein synthesis in the green alga Dunaliella tertiolecta

In the green marine alga Dunaliella tertiolecta, a CO2-concentrating mechanism is induced when the cells are grown under low-CO2 conditions (0.03% CO2). To identify proteins induced under low-CO2 conditions the cells were labelled with 35SO42-, and seven polypeptides with molecular weights of 45, 47, 49, 55, 60, 68 and 100 kDa were detected. The induction of these polypeptides was observed when cells grown in high CO2 (5% CO2 in air) were switched to low CO2, but only while the cultures were growing in light. Immunoblot analysis of total cell protein against pea chloroplastic carbonic anhydrase polyclonal antibodies showed immunoreactive 30-kDa bands in both high- and low-CO2-grown cells and an aditional 49-kDa band exclusively in low-CO2-grown cells. The 30-kDa protein was shown to be located in the chloroplast. Western blot analysis of the plasma-membrane fraction against corn plasma-membrane ATPase polyclonal antibodies showed 60-kDa bands in both high- and low-CO2 cell types as well as an immunoreactive 100-kDa band occurring only in low-CO2-grown cells. These results suggest that there are two distinct forms of both carbonic anhydrase and plasma-membrane ATPase, and that one form of each of them can be regulated by the CO2 concentration.