Alignment of the radical generating subunit of subclass Ic RNR. The proposed subclass Ic  (uppermost 10 sequences, in blue box) contains an unusual tyrosine to phenylalanine substitution (column in blue box). Subclass Ic hence does not harbour a stable protein radical [18,19]. Archaea are in a pale yellow box. From the alignment, it is apparent that a group of archaea (Halobacterium sp., H. salinarium, H. borinquense, H. utahensis and N. magadii) contains subclass Ic radical generating subunits, while another (H. lacusprofundi, H. mukohataei, N. pharaonis and Halophage AAJ2005) carry a normal tyrosyl radical-containing enzyme.
Lundin et al. BMC Evolutionary Biology 2010 10:383 doi:10.1186/1471-2148-10-383