Initiation factors and

Roy, A. L., Meisteremst, M., Pognonec, P., and Roeder, R. G. (1991). Cooperative interaction of an initiator-binding transcription initiation factor and helix-loop-helix activator USF. Nature 354 245-248. 

Figure 1.8 Translation of messenger RNA. The attachment of a ribosome to the mRNA involves protein initiation factors and the recognition of a particular base sequence, the start codon. A single mRNA can be simultaneously translated by more than one ribosome, forming a polyribosome. Synthesis occurs in the direction from the 5 end of messenger RNA to the 3 end. For further details of protein synthesis see Chapter 20.

Formation of a Basal Transcription Apparatus from General Initiation Factors and RNA Polymerase... 

A transcription-competent pre-initiation complex consisting of general transcription initiation factors and RNA polymerase II, can be reconstituted in the test tube from the individual components. As outlined in Fig. 1.31, efficient reconstitution requires a defined order for the addition of the individual components. 

FIGURE 27-22 Protein complexes in the formation of a eukaryotic initiation complex. The 3 and 5 ends of eukaryotic mRNAs are linked by a complex of proteins that includes several initiation factors and the poly(A) binding protein (PAB). The factors elF4E and elF4G are part of a larger complex called elF4F. This complex binds to the 40S ribosomal subunit. 

Initiation of protein synthesis involves formation of a complex between the 30S ribosomal subunit, mRNA, GTP, fMet-tRNAfMet, three initiation factors, and the 50S subunit GTP is hydrolyzed to GDP and P,. 

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. 

Hinnebusch, A. G., Involvement of an initiation factor and protein phosphorylation in translational control of GCN4 mRNA. Trends Biochem. Sci. 15 148-152, 1990. 

Eukaryotic ribosomes are larger (80S) and more complex than prokaryotic ribosomes (70S). Initiation is basically similar in prokaryotes and eukaryotes except that in eukaryotes at least nine initiation factors are involved (cf. three factors in prokaryotes), the initiating amino acid is methionine (cf. N-formylmethionine in prokaryotes), eukaryotic mRNAs do not contain Shine-Dalgarno sequences (so the AUG initiation codon is detected by the ribosome scanning instead), and eukaryotic mRNA is monocistronic (cf. some polycistronic mRNAs in prokaryotes). Initiation in eukaryotes involves the formation of a 48S preinitiation complex between the 40S ribosomal subunit, mRNA, initiation factors and Met-tRNA 61. The ribosome then scans the mRNA to locate the AUG initiation codon. The 60S ribosomal subunit now binds to form the 80S initation complex. 

In initiation of translation, the mcthionyl-tRNA species tRNAf 101 is used exclusively. Presumably, the unique nucleotide sequence of this species is required for the initial interaction with the small ribosomal subunit, the leader region of mRNA, initiation factors, and GTP to give the first initiation complex. It is unlikely that formylation (which can occur only with mcthionyl-tRNA el) is essential for initiation, as this is not the case in eukaryotes. It thus appears that tRNAjJJ - 1 is excluded from initiation because essential structural requirements in the RNA are not met. 

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... 

Matsushima, M., The role of L-tryptophan s promoting factor on tumorgenesis in the urinary bladder. 2. Urinary bladder carcinogenicity of FANFT (initiating factor) and L-tryptophan (promoting factor) in mice (author s transl., Japanese), Ipn. J. Urol., 68, 731, 1977. 

For mRNAs degraded In these deadenylation-dependent pathways, the rate at which they are deadenylated controls the rate at which they are degraded. The rate of deadenylation varies Inversely with the frequency of translation initiation for an mRNA the higher the frequency of Initiation, the slower the rate of deadenylation. This relation probably is due to the reciprocal interactions between initiation factors and PABPI that stabilize the binding of PABPI to the poly(A) tall, thereby protecting it from the deadenylation exonuclease. 

Prokaryotic initiation factors. In addition to the ribosomal proteins, the initiation factors IFl, IF2, and IF3, whose molecular masses are 9.5, 9.7, and 19.7 kDa, respectively, " are essential. They coordinate a sequence of reactions that begins with the dissociation of 70S ribosomes into their 30S and 50S subunits. Then, as is shown in Fig. 29-10, the mRNA, the initiator tRNA charged with formylmethionine, the three initiation factors, and the ribosomal subunits react to form 70S programmed ribosomes, which carry the bound mRNA and are ready to initiate protein synthesis. IF2 is a specialized G protein (Chapter 11), which binds and hydrolyzes GTP. It resembles the better known elongation factor EF-Tu (Section 2). The 172-residue IF3 consists of two compact a/p domains linked by a flexible sequence, which may exist as an a Its C-terminal domain binds to the central domain of the 16S RNA near nucleotides 819-859 (Fig. 

Fig. 2. Formation of a stable initiation complex between a 70 S ribosome and messenger RNA. In the final complex fMet-tRNAf " is in the correct position for the formation of a peptide bond. IF-1, IF-2, and IF-3 are the protein initiation factors and fMet-tRNAf " is the formyl methionyl tRNA which is used for the initiation of protein synthesis in prokaryotes. The process in animal cells is thought to be substantially the same, the initiation factors being termed IF-Ml, IF-M2, and IF-M3 and the initiator tRNA, Met-tRNAt . The methionine attached to this tRNA species is not normally formylated but can be so modified by enzymes from bacterial cells.

Post-transcriptional control of protein synthesis in bacteria seems to be confined to the possible existence of messenger or cistron-specific initiation factors and their modification after phage infection. In animal cells, however, where the site of transcription is physically separated by the nuclear membrane from the protein synthesizing apparatus in the cytoplasm, there is die possibility of more complex forms of control, and some of these will be discussed lielow. 

In the second step, the virus synthesizes an inhibitor that inactivates an initiation factor and by the mid-point of infection, most ribosomes are directed towards translating viral mENA. This two step process could explain the differential effect of such perturbations as exposure to high salt concentrations and histidinol treatment on cellular and viral mENA translation and the inactivation of initiation factor eIF-4B after infection of HeLa cells with poliovirus. 

In prokaryotes and eukaryotes, the initiation complex is prepared for the addition of the large ribosomal subunit by the release of initiation factor 3. In bacteria, the 508 subunit appears simply to replace IF-3, while IF-1 and IF-2 leave the complex afterward. In eukaryotes, another factor, eIF-5, catalyses the departure of the previous initiation factors and the joining of the 608 subunit. In both cases, release of initiation factor 2 involves hydrolysis of the GTP bound to it. Also, the Met-tRNA, is bound to the P site of the large ribosomal subunit. 

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