Fig. 1. Families within the cytochrome bd-type oxygen reductase superfamily.

The cytochrome-bd type oxygen reductase superfamily is divided into three families—qOR, OR-C and OR-N, primarily defined by the presence of the quinol binding site in the first, the presence of heme c binding site in the second and the abundance of OR-N enzymes in Nitrospirota. The above schematic represents the various subfamilies within each family that are defined by the phylogenetic clustering shown in Supplementary Fig. S1. The operon context and putative complex arrangement of each CydA-containing enzyme is also shown with a reference protein accession number and source microorganism. The potential gene duplication events are highlighted in yellow. A legend is also provided to mark the related conserved domains in the same colors and redox co-factors such as hemes and iron-sulfur clusters.