Carbonic anhydrase is essential for growth of Ralstonia eutropha at ambient CO(2) concentrations

Mutant strain 25-1 of the facultative chemoautotroph Ralstonia eutropha H16 had previously been shown to exhibit an obligately high-CO 2 -requiring (HCR) phenotype. Although the requirement varied with the carbon and energy sources utilized, none of these conditions allowed growth at the air concentration of CO 2 . In the present study, a gene designated can and encoding a β-carbonic anhydrase (CA) was identified as the site altered in strain 25-1. The mutation caused a replacement of the highly conserved glycine residue 98 by aspartate in Can. A can deletion introduced into wild-type strain H16 generated mutant HB1, which showed the same HCR phenotype as mutant 25-1. Overexpression of can in Escherichia coli and mass spectrometric determination of CA activity demonstrated that can encodes a functional CA. The enzyme is inhibited by ethoxyzolamide and requires 40 mM MgSO 4 for maximal activity. Low but significant CA activities were detected in wild-type H16 but not in mutant HB1, strongly suggesting that the CA activity of Can is essential for growth of the wild type in the presence of low CO 2 concentrations. The HCR phenotype of HB1 was overcome by complementation with heterologous CA genes, indicating that growth of the organism at low CO 2 concentrations requires sufficient CA activity rather than the specific function of Can. The metabolic function(s) depending on CA activity remains to be identified.