Overview RNR phylogenies. Unrooted BioNJ phylogenies from alignments of protein sequences of a) the catalytic subunit of class I RNR (NrdA and NrdE for subclass Ia and Ib respectively), b) the radical generating subunit of class I RNR (NrdB and NrdF for subclass Ia and Ib respectively), c) class II RNR (NrdJ) and d) the catalytic subunit of class III RNR (NrdD). Sequences were selected to cover the known sequence diversity. See inset colour legend for colours of the largest organismal groups. For black branches, see individual trees in additional file 1. Dashed branches are viruses; if the host range was known to us, viral sequences have the same colour as the host organism. Small arrows in a) and b) indicate the location of archaeal sequences. The marked parts of each tree were subjected to full maximum likelihood analyses, as indicated. All maximum likelihood trees are available in additional file 1; the trees from the marked areas of radical generating subunit of class I (NrdB, 1b) are only available in additional file 1.