Synapomorphy support in the 23S rRNA for the sister-group relationship between Paulinella and free-living α-cyanobacteria. Shown is the alignment and secondary structure diagram of Helix 837 in the 23S rDNA, with two RNA base pairs highlighted that represent synapomorphies of α-cyanobacterial clades to the exclusion of Paulinella and other prokaryotes. Sequence data and evolutionary changes are plotted on a simplified phylogram (NJ-bootstrap consensus tree). Pair 868/909 shows a uniquely derived CBC (compensatory base change: U-A → C-G) of all free-living α-cyanobacteria; the neighbouring pair 869/908 changed in the common ancestor of the marine PS-subclades (marine Synechococcus and Prochlorococcus) whereas the Cyanobium-clade and Paulinella are plesiomorphic. Pair 869/908 shows parallel changes in a few other cyanobacteria, (e.g. in Fischerella).