Translation in prokaryotes

Bacteria utilize N-formylmethionine (f-Met) rather than methionine (Met) at the N-terminal of the translated polypeptide.

N-Formylmethionine (fMet) is an amino acid that is found in all living cells. It is a derivative of the amino acid methionine, in which a formyl group has been added to methionine's amino group (catalyzed by the enzyme transformylase when Met is attached to tRNA.fMet, but not to tRNA.Met).

N-Formylmethionine is delivered to the 30S ribosome-mRNA complex by the specialized tRNA (tRNA.fMet), which possesses a 5'-CAU-3' anticodon that is capable of binding with the mRNA’s AUG start codon. N-Formylmethionine is coded for by AUG, which is the Met codon. However, AUG is also the translation initiation codon. When the AUG codon is employed for initiation, prokaryotes and plastids insert N-formylmethionine instead of methionine at the N-terminal of the nascent peptide chain. When the AUG codon appears farther along the mRNA, unmodified Met is inserted. Many organisms employ varations of this basic mechanism.

Thus, N-Formylmethionine plays a crucial role in the protein biosynthesis of bacteria, archaea, mitochondria and chloroplasts. However, f-Met is not employed in eukaryotic translation within the cytoplasm, where eukaryotic nuclear genes are translated into proteins.

Physical and Functional Interaction between the Eukaryotic Orthologs of Prokaryotic Translation Initiation Factors IF1 and IF2.
The fundamental process of translation initiation has been conserved between prokaryotes and eukaryotes. The initiator Met-tRNA is bound to the small ribosomal subunit, and this complex is localized to the AUG start codon of an mRNA. Three translation initiation factors (IFs) have been identified in prokaryotes (reviewed in references 7 and 18). Factor IF2 is responsible for binding fMet-tRNAiMet to the 30S ribosomal subunit. Factor IF1 binds to the 30S subunit and protects the same region of the ribosome (A site) as the elongation factor EF-Tu–GTP–aminoacyl-tRNA complex (28). Although a unique function has not been attributed to IF1, it does promote IF2 activities (reviewed in references 7, 18, 22, and 31. Factor IF3 dissociates ribosomal complexes, presumably to generate a pool of small ribosomal subunits for translation initiation. In addition, IF3 has recently been implicated in the process of ribosome recycling following termination of translation (21).

Sang Ki Choi, DeAnne S. Olsen, Antonina Roll-Mecak, Agnes Martung, Keith L. Remo, Stephen K. Burley, Alan G. Hinnebusch, and Thomas E. Dever Physical and Functional Interaction between the Eukaryotic Orthologs of Prokaryotic Translation Initiation Factors IF1 and IF2 Mol Cell Biol. 2000 October; 20(19): 7183–7191.

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