The degree of co-evolution was analysed by computing pairwise distances between sequence regions in the alignment of MT, AlkB, HEL and RdRp domains described above. In Figure 4 selected results are shown as scatter plots, where the Blosum 50 score value between e.g. the MT domains in a pair of sequences is plotted against the score value for AlkB domains in the same pair of sequences. Plots for the MT, HEL and RdRp domains show that they are strongly correlated for MT vs. RdRp (r 2 = 0.95), MT vs. HEL (r 2 = 0.87) and HEL vs. RdRp (r 2 = 0.81). The plot of the AlkB domain vs. these three domains for the same set of sequences shows a very low degree of correlation for AlkB vs. RdRp (r 2 = 0.10), AlkB vs. MT (r 2 = 0.12) and AlkB vs. HEL (r 2 = 0.16).