Once messenger has bound to the smaller ribosomal subunit and then a larger subunit has been added, translation can begin. Escherichia coli proteins initiate primarily at AUG codons with a peptide chain analog in which part of the initiating amino acid looks like a peptide bond (Fig. 1). This permits the system to form the first real peptide bond with essentially the same machinery as it uses to form subsequent peptide bonds.

Fig1. The structure of a polypeptide and the similar structure generated by formylation of an amino acid. The initiation amino acid is methionine, whose side group R is shown.

The initiation analog is N-formyl methionine. Rather than utilize an entirely separate charging pathway with a separate tRNA synthetase, the cells formylate a methionine after it has been put on a tRNA^Met. This is, however, a special tRNA called tRNA_f ^Met and it is used only for initiation. Methionine bound to the second type of tRNA^Met cannot be formylated, and this is used only for elongation at internal AUG codons. This scheme prevents the difficulties that would arise from attempting to initiate a protein with an unformylated methionine or attempting to put a formylated methionine on the interior of a protein. A new question is raised by the two tRNAs. How are the two methionine tRNAs distinguished during translation? AUG codons occur both at the beginning of protein coding sequences and within coding sequences.

The two Met-tRNAs are distinguished by a set of proteins used in the initiation and elongation processes. These proteins carry the charged tRNA molecules into the ribosome (Fig. 2). Interactions between the proteins and the ribosome-messenger complex help hold the tRNA-protein complex in the ribosome. The most important of these interactions is that between the three-base anticodon of the tRNA and the three-base codon in the messenger. It is these codon-anticodon interactions that specify the f-Met initiating amino acid and successive amino acids along the polypeptide chain. The protein IF2, initiation factor 2, carries the f−met−tRNA_f^Met into the site normally occupied by the growing peptide chain, the P site, whereas all other charged tRNAs, including met-tRNA, are carried into the other site, the acceptor, A, site, by the elongation factors. Thus N-formyl methionine can be incorporated only at the beginning of a polypeptide.

Fig2. Formation of a translation initiation complex.

In addition to the initiation factor IF2, two other proteins, IF1 and IF3, are also used during the initiation steps. Factor IF1 accelerates the initiation steps but is not absolutely required, and it can be assayed in vitro by its acceleration of ³H-Met-tRNA^Met binding to ribosomes. IF3 binds to the 30S subunit to assist the initiation process.

The initiation of translation in eukaryotes shows some similarity to the process used in bacteria. Two methionine tRNAs are used, one for initiation and one for elongation, but the methionine on the initiating tRNA is not formylated. Methionine on this tRNA, however, can be formylated by the bacterial formylating enzyme. It appears as if the eukaryotic initiation system has evolved from the bacterial system to the point that the formyl group is no longer necessary for protein synthesis, but the tRNA_f^Met is still similar to its progenitor bacterial tRNA_f^Met.

مواضيع ذات صلة

Translocation

Protein Elongation

Eukaryotic Translation and the First AUG

Experimental Support for the Shine-Dalgarno Hypothesis

تشكيل فُقَاعات التنسخ Replication Bubbles

ابتداء تخليق الدنا وتطويله Initiation & Elongation

تشكيل شعبة التنسخ Replication Fork

مناشئ التنسخ (Ori)

Regulation of RNA Stability by 5 and 3 UTR Sequences and Structures

Base Pairing between Ribosomal RNA and Messenger

Decoding the Message

How Synthetases Identify the Correct tRNA Molecule