(a) Maximum likelihood phylogeny of class I RNR radical generating subunit with subclasses in color, names to the left and organism distributions to the right (see inset legend; sizes of circles are proportional to the number of sequences found in each taxon). Bootstrap support values greater than 0.7 are shown with colored diamonds. NrdBs with ATP-cones were discovered in two subclasses: NrdBi and NrdF (formerly class Ib). Neither of the two subclasses have corresponding catalytic subunits, NrdAi and NrdE respectively, with ATP-cones. In both subclasses, NrdB sequences with ATP-cones were rare and phylogenetically limited, see inset arrows. (b) Arrangement of class I RNR genes in three genomes encoding NrdB proteins with ATP-cones (green borders, NrdAi/NrdBi; purple borders, NrdE/NrdF). Genes are shown 5’ to 3’, so that ATP-cones in the N-terminus are to the left in the gene.