Mitochondrial tyrosyl-tRNA synthetases : evolving a function beyond translation
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Pezizomycotina mitochondrial tyrosyl-tRNA synthetases (mtTyrRS) are bifunctional, with the ability to splice group I introns in addition to catalyzing aminoacylation. Work done with the Neurospora crassa mtTyrRS (CYT-18 protein) showed that it promotes splicing by binding and stabilizing the conserved catalytically active structure of the intron RNA. To interact with intron RNA, Pezizomycotina mtTyrRSs evolved a new intron-binding surface via structural adaptations on the side of the catalytic domain opposite that which binds tRNA[superscript Tyr]. To examine the variability of these adaptations and intron-binding surface differences between Pezizomycotina mtTyrRSs, I solved the structures of C-terminally truncated C. posadasii and A. nidulans mtTyrRSs. Comparison of these structures to CYT-18 revealed differences in some of the Pezizomycotina specific adaptations that are important for stabilizing key tertiary interactions required for group I intron folding. These studies highlight variations that likely affect intron-RNA binding and potentially splicing and also help define regions for therapeutic intervention. While my and previous studies have provided information on the N-terminal adaptations and intron-binding interactions, little information is available about the Cterminal domain (CTD) interactions. I conducted small angle X-ray scattering (SAXS), binding and splicing assays to further elucidate the domain arrangements of full length CYT-18 and contributions of the CTDs to splicing. My results suggest a model in which free CYT-18 exists in an extended conformation in solution, and upon binding intron RNA, forms a compact structure with both CTDs clamping down onto the RNA. These studies also revealed that the mtTyrRS CTDs have high non-specific binding affinity, which may have facilitated the evolution of the RNA splicing activity in Pezizomycotina mtTyrRSs. Finally, to further investigate the evolution of splicing activity by Pezizomycotina mtTyrRSs, which likely occurred during or after its divergence from Saccharomycotina, I studied bioinformatically reconstructed ancestral mtTyrRSs from the two fungal subphylums. These studies suggest that the common ancestor of the two subphylums may have been capable of non-specifically binding nucleic acid. My research suggests an evolutionary scenario in which an initial non-specific interaction between a self-splicing intron and an ancestral mtTyrRS led to the dependence of the intron on the mtTyrRS for splicing.