Primordial role for RNA aminoacylation in the assembly of structured and functional chimeric RNAs

Abstract

The origin of the universally conserved ribosomal machinery is one of the greatest mysteries of evolutionary biochemistry. Although many biochemical reactions participate in extant ribosomal translation, the most ancient function of the ribosome is to bring two aminoacylated RNA substrates into proximity and the correct orientation for peptide bond formation. This suggests that aminoacylation of RNA predates the ribosome, yet in the absence of catalysts this reaction is extremely inefficient. The prevailing view is that RNA catalysts, i.e. ribozymes, generated aminoacylated RNA. However, in the absence of ribosomal translation, it is unclear what evolutionary pressures could drive the evolution of ribozymes with aminoacylation function. My dissertation work has focused on understanding primordial roles for aminoacylated RNA that could have resulted in the selection of highly efficient aminoacyl-RNA synthetase ribozymes. We have discovered that aminoacylation of RNA enhances the nonenzymatic ligation reactions of RNA to generate chimeric molecules composed of RNA oligonucleotides bridged by amino acids. The assembly of functional chimeric ribozymes from aminoacylated RNA is facile even under mild conditions, while the assembly of RNA-only ribozymes requires a combination of nucleophilic catalysts and high concentrations of divalent metal ions. We have also discovered that RNA structure dictates the efficiency of the aminoacylation-facilitated assembly, with a universally conserved structural element in tRNA and rRNA, the T-loop motif, catalyzing the reaction to near completion. Such RNA structure-guided aminoacylation and assembly of chimeric ribozymes requires no external template, which subsequently enables autocatalytic assembly and unlimited propagation of chimeric aminoacyl-RNA synthetase ribozymes. Our work establishes RNA aminoacylation as crucial for the assembly of structured and functional RNA, reviving a potential primordial role for these ancient molecules that has been erased by the emergence of the ribosome.