Phylogeny of class I RNR catalytic subunit sequences shows a bacterial origin of eukaryote NrdA plus transfer from eukaryotes to bacteroidetes. A PhyML UL3 phylogeny from an alignment of protein sequences of the catalytic subunit of class I RNR from eukaryote genomes, and a selection of closely related bacterial homologues. Large top-level groups from the NCBI taxonomy have been colour-coded, see inset legend with a bar indicating eukaryotic groups. Smaller groups are black and viruses are in italics. The bacterial sequences (mainly Bacteroidetes) we suggest were horizontally transferred from eukaryotes have been indicated with a bar and label. The tree is not formally rooted; the pseudoroot is placed so as to be consistent with the global phylogeny in Figure 1a.