Initiation of polypeptide synthesis

The Three Steps of Initiation The initiation of polypeptide synthesis in bacteria requires (1) the 30S ribosomal subunit, (2) the mRNA coding for the polypeptide to be made, (3) the initiating fMet-tRNAfMet, (4) a set of three proteins called initiation factors (IF-1, IF-2, and IF-3), (5) GTP, (6) the 50S ribosomal subunit, and (7) Mg2+. Formation of the initiation complex takes place in three steps (Fig. 27-20). 

In addition to mRNA, fMet-tRNA tc , and the ribosome subunits, three initiation factors (proteins) and GTP are involved in the initiation of polypeptide synthesis. The process is described in Example 17.7. 

The first step of RNA translation begins with the initiation of polypeptide synthesis (Fig. 17-10). GTP is bound into the 30S initiation complex and is subsequently hydrolyzed and released upon binding to the 50S subunit. The fMet-tRNA eI occupies what is known as the peptidyl (P) site of the ribosome (Fig. 17-9) another site (A), capable of accommodating an aminoacyl-tRNA, is empty at this stage. It is aligned with the next codon (shown as xxx in Fig. 17-10) in the mRNA. 

Why can t tRNAm1 1 function in initiation of polypeptide synthesis from an appropriate AUG codon ... 

Codon AUG for Met also codes for the initiation of polypeptide synthesis. 

Yes. There are two different tRNAs for methionine, one involved in initiation and the other that adds methionine to the growing chain. The two species are called tRNA and tRNA. The former is capable of being formylated by a transformylase to yield A -formyMet-tRNA (or fMet-tRNA for short) and is the species that is involved exclusively in the initiation of polypeptide synthesis. Presumably, unique features of the structure of the tRNA in this case are... 

Initiation factors contribute to the ribosome complex with the messenger RNA and the initiator methionyl-tRNA. Elongation factors assist the binding of all the other tRNAs and the translocation reaction that must occur after each peptide bond is made. Termination factors recognize a stop signal and lead to the termination of polypeptide synthesis and the release of the polypeptide chain and the messenger from the ribosome. 

For which is a promoter needed initiation of RNA synthesis or of polypeptide synthesis ... 

The inventory of phylogenetically relevant components offered by ribosomes comprises no less than three RNA molecules and about fifty proteins. Aminoacyl-tRNA synthetases and protein factors assisting the initiation, elongation and termination reactions of polypeptide synthesis constitute further probes of potential usefulness to delineate the unfolding of the early lineages. 

Several possible explanations for these unusual molar ratios have been proposed. (1) There is more than one site of initiation of protein synthesis, one at the beginning of the gene coding for polypeptide A and one internally. Steric hindrance at the internal initiation site might affect the efficiency with which it is initiated, resulting in an abnormal A F C ratio. From our... 

Throughout the ribosome cycle, dynamic protein-mRNA interactions are functionally important in the initiation, elongation, and termination of polypeptide synthesis. In addition, more stable associations between proteins and mRNAs have been observed, particularly in eukaryotic cells. These messenger ribonucleoprotein complexes (mRNPs) occur both in polyribosomes and free in the cytosol, some of the latter being either temporarily or permanently unavailable for translation. Thus, protein-mRNA interactions contribute to the efficiency with which mRNAs are translated. 

Analogues of methionine merit a special interest because the biological functions of methionine (Met) in the cell are multiple (1) it is the main methyl group donor via S-adenosylmethionine (2) it plays a key rdle in the initiation of polypeptide chain synthesis as N-formylmethionine-(transfer RNA) and (3) the methyla-tion of transfer-RNAs bases may be important in conferring the specific coding properties (see Sect B. Para. 8.1.2). 

GTP hydrolysis must have occurred. Indeed, nonhydrolyzable analogues of GTP block joining of the 60 S subunit (Trachsel et al., 1977 Benne and Hershey, 1978). These reactions require the presence of eIF-5, the 60 S joining factor (160,000 Mr) (Trachsel et al., 1977 Benne and Hershey, 1978). The resulting 80 S initiation complex is apparently free of initiation factors, and its Met-tRNAf is reactive to puromycin, meaning that it is capable of entering the elongation phase of polypeptide synthesis. 

Initiation of protein synthesis in mammalian cells proceeds by a complex process whereby the assembly of mRNA, the ribosome, and initiator met-tRNAf into an initiation complex is catalyzed by a group of proteins called initiation factors. By definition, these proteins are not required for polypeptide chain elongation. A detailed discussion of this process, and the role of individual initiation factors can be found in this volume (Kaempfer, 1984), or in other recent reviews (Benne and Hershey, 1978 Jagus et al., 1981) which also contain pertinent references. Nine initiation factors have been highly purified from rabbit reticulocytes, and still other factors have been described which may serve auxiliary functions, or indeed may qualify as initiation factors in their own right. A very brief (and oversimplifed) description of the role of the nine, characterized initiation factors is listed below, along with the step in initiation complex formation in which each participates ... 

The information contained in the base sequence of the mRNA template is interpreted in sequences of three bases called codons each codon represents one amino acid. Therefore, the unit of information is the codon. Since there are four major bases in mRNA, 4 (i.e. 64) different codons are possible. The 64 triplets constitute the genetic code (Table 17.1). All codons have been assigned to amino acids or punctuation signals. Three triplets (UAA, UAG and UGA) are not complemented by anticodons on tRNAs and serve to signal that the polypeptide chain has been completed. Of the other 61 triplets which have complementary tRNAs, two (AUG and GUG) have additional roles in the initiation of protein synthesis. Since there are only 20 amino acids, most amino acids are specified by more than one codon, i.e. the code is degenerate. The genetic code applies to prokaryotes and eukaryotic nuclear and chloroplast mRNAs but not to... 

---