9.A.2 The Putative Dissolved Inorganic Carbon Concentrating Transporter (DIC-CT) Family 

Many autotrophic microorganisms adapt to scarcity in dissolved inorganic carbon (DIC; = CO² + HCO₃^- + CO₃^2-) with CO₂ concentrating mechanisms (CCM) which actively transport DIC across the cell membrane to facilitate carbon fixation. The deep-sea vent γ-proteobacterial chemolithoautotroph, Thiomicrospira crunogena, has a low-DIC inducible CCM. Proteins encoded by Tcr_0854 and the adjacent Tcr_0853 genes, required elevated DIC for growth (Mangiapia et al. 2017). Strains carrying mutated copies of Tcr_0853 and Tcr_0854 overexpressed carboxysomes and had diminished ability to accumulate intracellular DIC. Tcr_0853 and Tcr_0854 are cotranscribed and upregulated under low-DIC conditions. The Tcr_0853 protein has 13 - 15 TMSs and is homologous to NADH dehydrogenase subunits (TC#3.D.1). Tcr_0853 and Tcr_0854 may encode a two-subunit DIC transporter. These proteins are widespread among autotrophs from multiple phyla of Bacteria and Archaea. Homologues are present in 14 phyla in Bacteria and in one phylum of Archaea, the Euryarchaeota. Many organisms carrying these homologs are autotrophs, suggesting a role in facilitating dissolved inorganic carbon (Mangiapia et al. 2017).

DabB2/DabA2 comprises a putative CO₂ concentrating transporter, where DabB2 is a possible inorganic ion (H⁺ or Na⁺) transporter, homologous to integral membrane NADH dehydrogenase subunites, 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 (DabA and DabB), for DABs accumulate bicarbonate - assemble into a heterodimeric complex, which 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. Possibly DABs constitute a class of energized C_i pumps and play a critical role in the metabolism of C_i throughout prokaryotic phyla.