Figure 8. Model for the evolution of the RNA splicing activity of fungal mtTyrRSs via non-adaptive non-specific RNA-binding by the CTD.

(A) A Pezizomycotina ancestor contained a self-splicing group I intron and an mtTyrRS, which functions in aminoacylation by specifically binding tRNA^Tyr. (B) The ancestral mtTyrRS could bind non-specifically to the group I intron and other RNAs via its CTD, and its interaction with a group I intron RNA was fixed by an intron RNA mutation (orange star) that made it dependent upon the binding of the mtTyrRs for formation or stabilization of the active ribozyme structure. (C) Once fixed, this initial non-specific interaction was elaborated by further adaptive mutations in both the protein (orange spheres) and intron RNA (orange stars) that increased both the efficiency and protein-dependence of RNA splicing. The adaptive mutations in the fungal mtTyrRSs resulted in specific binding of the conserved tertiary structure of the group I intron RNA catalytic core, conferring the ability to bind and splice additional group I introns (blue, pink).