Translation initiation with initiator tRNA charged with exotic peptides.

In the prokaryotic translation system formylmethionyl-tRNA(fMet)(CAU) acts as an exclusive initiator to yield peptides bearing formylmethionine at the N-terminus. We herein report a new methodology to initiate the translation reaction with peptidyl-tRNA(fMet)(CAU) in which the peptidyl group consists of unusual building blocks, such as D-amino acids, beta-amino acids, and N-methyl amino acids, and express peptides containing a stretch of exotic peptides at the N-terminus. Synthesis of the initiator peptidyl-tRNAs was facilitated by flexizyme, a flexible tRNA aminoacylation ribozyme, and the start codon was reprogrammed by withdrawing methionine from a reconstituted E. coli cell-free translation system to reassign the initiator from formylmethionine to exotic peptides. This represents the first demonstration for initiating the ribosomal peptide synthesis with peptides, which offers us a new tool for the synthesis of a wide variety of unusual peptides and also the mechanistic studies on the initiation and/or elongation events in translation.