Figure 3. Multiple horizontal gene transfer (HGT) events involved in the dispersal of the nqr operon.

Our data indicate that Na⁺-NQR appeared in the common ancestor of the Chlorobi and Bacteroidetes groups. The operon was transferred by HGT to Chlamydiae and Planctomyces, and to the ancestor of α, β, γ and δ–proteobacteria, after this lineage split from ε -proteobacteria. The evidence indicates that both Desulfobacteraceae (δ–proteobacteria) and the ancestor of Burkholderales, Hydrogenophilales and Neisseriales (β–proteobacteria), lost the nqr operon and was reacquired later. In the case of Desulfobacterales, from Chlorobi, and in the case of Neisseriales, from pathogenic Pasteurelalles (γ–proteobacteria).