INDUCTION OF INTRACELLULAR CARBONIC-ANHYDRASES DURING THE ADAPTATION TO LOW INORGANIC CARBON CONCENTRATIONS IN WILD-TYPE AND CA-1 MUTANT-CELLS OF CHLAMYDOMONAS-REINHARDTII

Mass-spectrometric measurements of O-18 exchange from (CO2)-C-13-O-18 were used to follow changes in the intracellular carbonic anhydrase (C A) activity of cells of Chlamydomonas reinhardtii Dang. wild type and the ca-1 mutant during adaptation to air. With intact cells as well as with crude homogenates total intracellular CA activity in wild-type c ells increased six to tenfold within 4 h after transferring cells from 5% CO2 (high inorganic carbon, C-i) to ambient air (air adapted). Aft er that time the activity slowly declined to a level similar to that o bserved with cells which had been continuously grown in air (low-C-i g rown). In the ca-1 mutant, total CA was induced to a similar extent du ring 4 h of adaptation; however, absolute activities were two to three times lower in ca-1 than in the wild type regardless of the CO2 suppl y. When crude extracts from wild-type cells were separated into solubl e and insoluble fractions, each fraction contained about half of the i nternal CA activity. Within 4 h of adaptation, both forms of CA activi ty were simultaneously enhanced by nine to tenfold, reaching levels si milar to those found in low-C-i-grown cells. In contrast, in the ca-1 mutant the soluble CA activity was only enhanced by about eight-fold w hile the level of insoluble CA was very low even in low-C-i cells. Aft er isolation of intact chloroplasts from wild-type cells and further s ubfractionation, around 70-80% of total chloroplastic CA activity was found to be in the insoluble fraction while 17-20% remained in the sol uble fraction. Both chloroplastic CA activities were inducible within the first 4 h of adaptation to air, with each of them being eight to t en times higher than in high-C-i algae. After that time their activiti es were similar to the corresponding CA values in low-C-i-grown cells. In contrast, plastids from high-C-i cells of the ca-1 mutant showed 4 0% less insoluble-CA activity compared to the wild type and this insol uble-CA activity was not increased at all by transferring algae to air . In addition, no soluble-CA activity was detected in chloroplasts fro m high-C-i and air-adapted ca-1 cells. These results indicate the pres ence of three intracellular CA activities in high-C-i air-adapted and low-C-i cells of the wild type and that two of them are associated wit h the chloroplasts. All three activities are completely induced within the first 4 h of adaptation to air in wild-type cells. In contrast, i t was not possible to induce any of the chloroplastic CA activities in the ca-1 mutant. The possibility that the soluble chloroplastic CA re presents a pyrenoid-located CA is discussed.