Initiation factors ribosome associated

Two initiation factors, eIF-3 and elF-lA, bind to the newly dissociated 40S ribosomal subunit. This delays its reassociation with the 60S subunit and allows other translation initiation factors to associate with the 40S subunit. 

Figure 7.5 Model of ferritin (and erythroid a-aminolaevulinate synthase) translation/ribosome binding regulation by IRP. In (a), with IRP not bound to the IRE (1) binding of the 43S preinitiation complex (consisting of the small ribosomal 40S subunit, GTP and Met-tRNAMet) to the mRNA is assisted by initiation factors associated with this complex, as well as additional eukaryotic initiation factors (elFs) that interact with the mRNA to facilitate 43S association. Subsequently (2), the 43S preinitiation complex moves along the 5 -UTR towards the AUG initiator codon, (3) GTP is hydrolysed, initiation factors are released and assembly of the 80S ribosome occurs. Protein synthesis from the open reading frame (ORF) can now proceed. In (b) With IRP bound to the IRE, access of the 43S preinitiation complex to the mRNA is sterically blocked. From Gray and Hentze, 1994, by permission of Oxford University Press.

Lomakin, I. B., Hellen, C. U., and Pestova, T. V. (2000). Physical association of eukaryotic initiation factor 4G (eIF4G) with eIF4A strongly enhances binding of eIF4G to the internal ribosomal entry site of encephalomyocarditis virus and is required for internal initiation of translation. Mol. Cell Biol. 20, 6019-6029. 

Abbreviations aa-tRNA Amino-acyl tRNA eLF Eukaryotic translation initiation factor IF Prokaryotic translation initiation factor eEF Eukaryotic translation elongation factor EF Prokaryotic translation elongation factor eRF Eukaryotic translation termination factor (release factor) RF Prokaryotic translation release factor RRF Ribosome recycling factor Rps Protein of the prokaryotic small ribosomal subunit Rpl Protein of the eukaryotic large ribosomal subunit S Protein of the prokaryotic small ribosomal subunit L Protein of the prokaryotic large ribosomal subunit PTC Peptidyl transferase center RNC Ribosome-nascent chain-mRNA complex ram Ribosomal ambiguity mutation RAC Ribosome-associated complex NMD Nonsense-mediated mRNA decay... 

Tapprich WE, Dahlberg AE (1990) A single base mutation at position 2,661 in E. coli 23S ribosomal RNA affects the binding of ternary complex to the ribosome. EMBO J 9 2649—2655 Tarun Jr SZ, Sachs AB (1996) Association of the yeast poly(A) tail binding protein with translation initiation factor eIE-4G. EMBO J 15 7168-7177... 

Initiation The components of the translation system are assembled, and mRNA associates with the small ribosomal subunit. The process requires initiation factors. In prokaryotes,a purine-rich region (the Shine-Dalgarno sequence) of the mRNA base-pairs with a complementary sequence on 16S rRNA, resulting in the positioning of the mRNA so that translation can begin. The 5 -cap on eukaryotic mRNA is used to position that structure on the ribosome. The initiation codon is 5 -AUG-3. ... 

Even though specific differences distinguish the initiation process in eukaryotes and prokaryotes, three things must be accomplished to initiate protein synthesis in all systems (1) The small ribosomal subunit must bind the initiator tRNA (2) the appropriate initiating codon on mRNA must be located and (3) the large ribosomal subunit must associate with the complex of the small subunit, the initiating tRNA, and mRNA. Nonribosomal proteins, known as initiation factors (IFs), participate in each of these three processes. IFs interact transiently with a ribosome during initiation and thus differ from ribosomal proteins, which remain continuously associated with the same ribosome. 

E. coli has three initiation factors (fig. 29.13) bound to a small pool of 30S ribosomal subunits. One of these factors, IF-3, serves to hold the 30S and 50S subunits apart after termination of a previous round of protein synthesis. The other two factors IF-1 and IF-2, promote the binding of both fMet-tRNA,Mel and mRNA to the 30S subunit. As we noted before, the binding of mRNA occurs so that its Shine-Dalgarno sequence pairs with 16S RNA and the initiating AUG sequence with the anticodon of the initiator tRNA. The 30S subunit and its associated factors can bind fMet-tRNAjMet and mRNA in either order. Once these ligands are found, IF-3 dissociates from the 30S, permitting the 50S to join the complex. This releases the remaining initiation factors and hydrolyzes the GTP that is bound to IF-2. The initiation step in prokaryotes requires the hydrolysis of one equivalent of GTP to GDP and Pj. 

Translation begins with the binding of the ribosome to mRNA. A number of protein factors transiently associate with the ribosome during different phases of translation Initiation factors, elongation factors, and termination factors. 

Several protein factors are involved in the initiation process. These factors aren t usually part of the ribosome instead, they help form an active initiation complex. Initiation factor 3 (IF3) helps keep the 30S subunit dissociated from the 50S subunit and available for protein synthesis. IF1 binds to the isolated 30S subunit and helps form the complex between the RBS and 16S rRNA. IF2 forms a complex with frnet-tRNAme i and GTP, releasing IF3. After the complex contains mRNA and initiator fmet-tRNA, the following things occur GTP is hydrolyzed to GDP, the initiation factors are released from the ribosome, and the 50S subunit associates with the complex to form an elongating ribosome, as shown in Figure 11-5. 

Translation in prokaryotes begins with the formation of the ribosome complex at a defined position on the mRNA, termed the Shine-Delgamo sequence, or the ribosome binding site (RBS). In prokaryotic mRNA, this is a relatively small (4-7 nucleotides) region rich in purine nucleotides located less than 10 nucleotides to the 5 side of the translational start site (Fig. 23-3). This Shine-Delgarno sequence is complementary to the 16S ribosomal RNA (rRNA) associated with the 30S ribosomal subunit, and directs it to bind the mRNA at that position. The 3 OS ribosomal subunit will not bind this region on the mRNA without the aid of an associated protein called initiation factor 3 (IF-3). The 30S ribosomal subunit will bind the mRNA in such a manner that the peptidyl (P) site of the complex is occupied by a specialized codon with the sequence, AUG. It is at this AUG start codon where translation will eventually begin. 

Initiation. Protein synthesis in bacteria begins by the association of one 308 subunit (not the 708 ribosome), an mRNA, a charged tRNA , three protein initiation factors, and guanosine 5 -triphosphate (GTP). These molecules make up the 308 preinitiation complex. Association occurs at an initiator AUG codon, whose selection was described above. A 508 subunit joins to the 308 subunit to form a 708 initiation complex (Figure 25-11). This joining process requires hydrolysis of the GTP contained in the 308 preinitiation complex. There are two tRNA... 

Eukaryotes utilize many more initiation factors than do prokaryotes, and their interplay is much more intricate. The prefix elF denotes a eukaryotic initiation factor. For example, eIF-4E is a protein that binds directly to the 7-inethylguanosine cap (p. 846), whereas eIF-2, in association with GTP, delivers the met-tRNA to the ribosome. The difference in initiation mechanism between prokaryotes and eukaryotes is, in part, a conseciuence of the ence in RNA processing. The 5 end of mRNA is readily available to ribosomes immediately after transcription in prokaryotes. In contrast, pre-mRNA must be processed and transported to the cytoplasm in eukaryotes before translation is initialed. The 5 cap provides an easily recognizable starting point. In addition, the complexity of eukaryotic translation initiation provides another mechan ism for regulation of gene expression that we shall explore further in Chapter 31. 

Let us look more cl osely at the molecular events at the ribosome, where the recognition and formation of peptide bonds actually occurs. We have seen that the ribosome is composed of two different subunits, but only one of these subunits, the smaller of the two, is essential for initiation of protein synthesis, although it must be associated with the larger unit before chain elongation can proceed. Initiation also requires the presence of an energy source (supplied not by ATP but GTP), a particular amino acyl t-RNA whose anti-codon corresponds to the start here codon on m-RNA and, at least in bacteria, three soluble protein initiation factors called IF1, IF2, IF3. The ribosome has two sites for t-RNA binding, the P site and the A site, but only initiator t-RNA can bind to the P site - all other incoming amino acyl t-RNAs bind to the A site. 

Step Association of the large subunit (605) forms an SOS ribosome ready to translate the mRNA. Two initiation factors, elF2 (stepO) and elF5 (stepH) are GTP-binding proteins, whose bound GTP is hydrolyzed during translation initiation. The precise time at which particular initiation factors are released is not yet well characterized. See the text for details. [Adapted from R. Mendez and J. D. Richter, 2001, Nature Rev. Mol. Cell Biol. 2 521.]... 

It has been shown that the first step in ribosome-dependent peptide synthesis is activation of amino acids to form amino acid adenylates. The amino acids are then transferred to RNA present in the soluble extract of the cell, the so-called transfer RNA (tRNA) to which the amino acids become fixed by an ester linkage. These two steps are usually referred to as the formation of aminoacyl-tRNA. The next step, the translation step of codons in messenger RNA (mRNA), which is associated with ribosomes, to provide a polypeptide includes three stages (1) chain initiation by mutual coordination with initiation factors, (2) chain elongation in aid of elongation factors, and (3) chain termination in support of release factors. 

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