Phylogenies of class I RNR catalytic subunits show independent transfers from bacteria to archaea. PhyML UL3 phylogenies showing a) a probable bacterial origin of NrdA subunits from the archaea Halomicrobium mukohataei DSM 1228, Halorubrum lacusprofundi ATCC 49239 and Natronomonas pharaonis DSM 2160, and b) a probable bacterial origin of NrdA subunits from the archaea Halobacterium sp. NRC-1, Halobacterium salinarum R1, Halogeometricum borinquense DSM 11551, Halorhabdus utahensis DSM 12940 and Natrialba magadii ATCC 43099. Arrows point to the position of the archaeal sequences. Large top-level groups from the NCBI taxonomy have been colour-coded, see inset legend, and smaller groups are in black. Viruses are in italics. The tree is not formally rooted; the pseudoroot is placed so as to be consistent with the global phylogeny in Figure 1a.