the list is based on the data available at genomenet web site http://www.genome.jp/. click here for additional data file .
 carbonic anhydrase ( ca ) ( e.c . 
 4.2.1.1 ) is a ubiquitous enzyme catalysing interconversion between co2 and bicarbonate . 
 the irregular distribution of the phylogenetically distinct classes of ca in procaryotic genome suggests its complex evolutionary history in procaryotes . 
 genetic evidence regarding the dispensability of ca under high - co2 air in some model organisms indicates that ca - deficient microorganisms can persist in the natural environment by choosing high - co2 niches . in this study 
 , we studied the distribution of ca in the genome of proteobacteria . while a large majority of the genome - sequenced proteobacteria retained a ca gene(s ) , intracellular bacterial genera such as buchnera and rickettsia contained ca - defective strains . 
 comparison between ca - retaining and ca- deficient genomes showed the absence of whole coding sequence in some strains and the presence of frameshifted coding sequence in other strains . 
 the evidence suggests that ca is inactivated and lost in some proteobacteria during the course of evolution based on its dispensability . 
Supplementary Material