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The can (previously yadF) gene of Escherichia coli encodes a ␤-class carbonic anhydrase (CA), an enzyme which interconverts CO 2 and bicarbonate.Various essential metabolic processes require either CO 2 or bicarbonate and, although carbon dioxide and bicarbonate spontaneously equilibrate in solution, the low concentration of CO 2 in air and its rapid diffusion from the cell mean that insufficient bicarbonate is spontaneously made in vivo to meet metabolic and biosynthetic needs. We calculate that demand for bicarbonate is 10 3 -to 10 4 -fold greater than would be provided by uncatalyzed intracellular hydration and that enzymatic conversion of CO 2 to bicarbonate is therefore necessary for growth. We find that can expression is ordinarily required for growth in air. It is dispensable if the atmospheric partial pressure of CO 2 is high or during anaerobic growth in a closed vessel at low pH, where copious CO 2 is generated endogenously. CynT, the single E. coli Can paralog, can, when induced with azide, replace Can; also, the ␥-CA from Methanosarcina thermophila can at least partially replace it. Expression studies showed that can transcription does not appear to respond to carbon dioxide concentration or to be autoregulated. However, can expression is influenced by growth rate and the growth cycle; it is expressed best in slow-growing cultures and at higher culture densities. Expression can vary over a 10-fold range during the growth cycle and is also elevated during starvation or heat stress.

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