Plant Physiology 91:505-513 (1989)
© 1989 American Society of Plant Biologists
Metabolism and Enzymology
Expression of Human Carbonic Anhydrase in the Cyanobacterium Synechococcus PCC7942 Creates a High CO2-Requiring Phenotype 1
Evidence for a Central Role for Carboxysomes in the CO2 Concentrating Mechanism
G. D. Price and
M. R. Badger
Plant Environmental Biology Group, Research School of Biological Sciences, Australian National University, P.O. Box 475, Canberra, A.C.T. 2601 Australia
Active human carbonic anhydrase II (HCAII) protein was expressed in the cyanobacterium Synechococcus PCC7942 by means of transformation with the bidirectional expression vector, pCA. This expression was driven by the bacterial Tac promoter and was regulated by the IacIQ repressor protein, which was expressed from the same plasmid. Expression levels reached values of around 0.3% of total cell protein and this protein appeared to be entirely soluble in nature and located within the cytosol of the cell. The expression of this protein has dramatic effects on the photosynthetic physiology of the cell. Induction of expression of carbonic anhydrase (CA) activity in both high dissolved inorganic carbon (Ci) and low Ci grown cells leads the creation of a high Ci requiring phenotype causing: (a) a dramatic increase in the K0.5 (Ci) for photosynthesis, (b) a loss of the ability to accumulate internal Ci, and (c) a decrease in the lag between the initial Ci accumulation following illumination and the efflux of CO2 from the cells. In addition, the effects of the expressed CA can largely be reversed by the carbonic anhydrase inhibitor ethoxyzolamide. As a result of the above findings, it is concluded that the CO2 concentrating mechanism in Synechococcus PCC7942 is largely dependent on (a) the absence of CA activity from the cytosol, and (b) the specific localization of CA activity in the carboxysome. A theoretical model of photosynthesis and Ci accumulation is developed in which the carboxysome plays a central role as both the site of CO2 generation from HCO3 and a resistance barrier to CO2 efflux from the cell. There is good qualitative agreement between this model and the measured physiological effects of expressed cytosolic CA in Synechococcus cells.
1 This work was supported by a National Research Fellowship (G. D. P.) awarded by the Australian Department of Education Employment and Training.
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