Figure 1.

(Left) The three possible schemes that may have given rise to the extant distribution of class I LysRS enzymes within the phylogenetic tree. (Right) The predicted phylogenetic relationships that would be expected between class I LysRS sequences and their closest class I aaRSs. (Top) A late duplication event followed by horizontal transfer. (Middle) Two independent duplication and gene replacement events. (Bottom) Two ancestral enzymes coexist initially, and they displace each other in different groups. ○, Class II LysRS. •, Class I LysRS. ▴ ■, Class I synthetases closest to class I LysRS. ⊗, Displacement of class II LysRS.