Physiological and Molecular Studies on the Response of Cyanobacteria to Changes in the Ambient Inorganic Carbon Concentration

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The ability of cyanobacteria to adapt to a wide range of ambient CO2 concentrations involves modulation of the activity of an inorganic carbon-concentrating mechanism (CCM), as well as other changes at various cellular levels including the biosynthetic pathway of purines. Studies of high-CO2-requiring mutants have identified several of the genes involved in the operation of the CCM and in the ability to grow under changing ambient CO2 concentration. In the case of Synechococcus sp. strain PCC 7942 most of these genes have been mapped in the genomic region of the rbcLS operon. Higher levels of detectable transcripts originating from some of these genes have been observed after exposure of the cells to low CO2 concentration. Studies of mutants have confirmed quantitative models postulating crucial roles for carboxysomes and carboxysome-located carbonic anhydrase (CA) in cyanobacterial photosynthesis. A central role is also proposed for cytoplasmic-membrane-associated CA activity: CA may function to scavenge escaping CO2 by intracellular conversion to bicarbonate against the chemical potential.