20. Met-tRNA Met recognizes the initiating AUG codon in almost all cells It was observed that about half of the n-terminal residues of proteins in E coli are met, and the n-terminal of nascent polypeptides is usually modified There are two tRNAs for recognizing AUg and Met in all organisms: one(tRNA Met) brings Met to the initiating AUG and the other(tRNA Met) brings met to internal AUgs The Met charged on tRNAMet(by Met-tRNA synthetase is specifically formylated by a transformylase to form fMet-tRNAMet in bacteria(Met-tRNA Met can not be formylated) fMet-tRNAtMet can only enter and only fMet-tRNAMet can enter the initiation site(site P)on the ribosome in20. Met-tRNAi Met recognizes the initiating AUG codon in almost all cells • It was observed that about half of the N-terminal residues of proteins in E.coli are Met, and the N-terminal of nascent polypeptides is usually modified. • There are two tRNAs for recognizing AUG and Met in all organisms: one (tRNAi Met) brings Met to the initiating AUG and the other (tRNAMet) brings Met to internal AUGs. • The Met charged on tRNAi Met (by Met-tRNA synthetase) is specifically formylated by a transformylase to form fMet-tRNAfMet in bacteria (Met-tRNAMet can not be formylated). • fMet-tRNAfMet can only enter and only fMet-tRNAfMet can enter the initiation site (site P) on the ribosome in E.coli