9.A.2.1.2
DabB2/DabA2 putative CO₂ concentrating transporter, where DabB2 is a possible inorganic ion (H⁺ or Na⁺) transporter, homologous to integral membrane NADH dehydrogenase subunits, while DabA2 is homologous to carbonic anhydrases.  Bacterial autotrophs often rely on CO₂ concentrating mechanisms (CCMs) to assimilate carbon. Desmarais et al. 2019 performed a genome-wide barcoded transposon screen to identify essential and CCM-related genes in the γ-proteobacterium Halothiobacillus neapolitanus. Screening revealed that the CCM comprises at least 17 and probably no more than 25 genes, most of which are encoded in 3 operons. Two of these operons (DAB1 and DAB2) contain a two-gene locus that encodes a domain of unknown function (Pfam: PF10070) and a putative cation transporter (Pfam: PF00361). Physiological and biochemical assays demonstrated that these proteins (named DabA and DabB for "DABs accumulate bicarbonate") - assemble into a heterodimeric complex that contains a putative β-carbonic anhydrase-like active site and functions as an energy-coupled inorganic carbon (C_i) pump. DAB operons are found in a diverse range of bacteria and archaea. Desmarais et al. 2019 demonstrated that functional DABs are present in the human pathogens, Bacillus anthracis and Vibrio cholerae. They proposed that DABs constitute a class of energized C_i pumps and play a critical role in the metabolism of C_i throughout prokaryotic phyla. Since the uptake of Ci is inhibited by the ionophore CCCP, DAB2 activity may be energetically coupled to a cation gradient, and as DabA2–DabB2 shows pH-independent rescue of carbonic anhydrase-free E. coli, CO₂ is probably the transported substrate. Holthuijzen et al 1987 (DOI 10.1007/BF00463489) reported similar findings for Halothiobacillus neapolitanus, and (Menning et al. 2016) got similar results for Hydrogenovibrio crunogenus (Thiomicrospira crunogena). Both DAB operons in Halothiobacillus are required; each having an HCR phenotype when knocked out. A double deletion, which might be expected to have a more severe phenotype, was not constructed.