Codons initiation

Pig. 3. Representation of promoter sites on the pro-enkephalin gene. The numbers represent the distance in nucleotides from the pro-enkephalin initiation codon the arrow indicates the direction of transcription. The TATA promoter box occurs immediately before the pro-enkephalin initiation site the AP-2 site, which binds immediate-early gene products, is 70 nucleotides upstream, and the CRE site, which binds a regulatory protein involved in cAMP induction of mRNA synthesis, is 107 nucleotides upstream from the initiation codon. The expanded section shows that the CRE site actually consists of two elements, ENKCRE-1 and ENKCRE-2, which separately confer cAMP sensitivity to pro-enkephalin mRNA synthesis. 

Very recently, a third hypothesis has been pubHshed. Morita and co-workers [47] have suggested that the rpoH mRNA secondary structure itself acts as a thermosensor. In the absence of heat stress, the rpoH mRNA is folded into a secondary structure that occludes the ribosome binding site and the initiation codon. Upon heat shock, this structure is unfolded allowing ribosome binding and enhanced synthesis. 

The terms first, second, and third nucleotide refer to the individual nucleotides of a triplet codon. U, uridine nucleotide C, cytosine nucleotide A, adenine nucleotide G, guanine nucleotide Term, chain terminator codon. AUG, which codes for Met, serves as the initiator codon in mammalian cells and encodes for internal methionines in a protein. (Abbreviations of amino acids are explained in Chapter 3.)... 

The first step in this process involves the binding of GTP by eIF-2. This binary complex then binds to met-tRNAf a tRNA specifically involved in binding to the initiation codon AUG. (There are two tRNAs for methionine. One specifies methionine for the initiator codon, the other for internal methionines. Each has a unique nucleotide sequence.) This ternary complex binds to the 40S ribosomal subunit to form the 43S preinitiation complex, which is stabilized by association with eIF-3 and elF-lA. 

Fig. 8.4 Outline of the main events in protein synthesis initiation, elongation, translocation and termination. AUG is an initiation codon on the mRNA it codes for Af-fomiylmelhionine and initiates the formation of the 70S rihosome. UAG is a termination codon it does not code for any amino acid and brings about termination of protein synthesis.

Fig. 24.6 The use of a vector carrying a promoter and adjacent ribosome binding site (RBS) and initiation codon to obtain synthesis of proinsulin from a synthetic gene. The arrow indicates the direction of transcription.

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.

Kolupaeva, V.G., Fomakin, I.B., Pestova, T.V., and Hellen, C.U.T. (2003) Eukaryotic initiation factors 4G and 4A mediate conformational changes downstream of the initiation codon of the encephalomyocar-ditis virus internal ribosomal entry site. Mol. Cell. Biol. 23, 687-698. 

ALTERNATIVE START SITES If all of the above didn t provide enough diversity, some messages contain two AUG initiation codons separated by some intervening information. Protein synthesis can initiate at either site. This is useful for making proteins with or without NH2- terminal signal sequences. 

G-CSF expression is controlled at both the transcriptional and posttranscrip-tional levels. A sequence of 300 nucleotides upstream of the initiation codon is conserved in both the murine and human genes, and this appears to contain three regulatory sites. G-CSF (and some other cytokine genes) may be constitutively transcribed by cells such as blood monocytes, fibroblasts and endothelial cells, but the mRNA may be short-lived (fi/2 < 15 min). The mRNA contains poly-AUUUA sequences in the untranslated region, and this motif is usually associated with mRNA instability. Indeed, such regions have also been identified in mRNA for GM-CSF, IL-1, IL-6, interferons, TNF, some growth factors, c-jun, c-fos, c-myc and c-myb. Upon the addi-... 

Diester methods have been used to synthesize analogues of the initiator codon ApUpG in which the adenine residue has a fixed torsion angle, as for instance in (78),132 and triester methods have been used to prepare dinucleoside phosphates and codon analogues133 134 containing the hydroxyalkyl nucleosides 9-(4 -hydroxybutyl)-adenine (80), 9-(3 -hydroxypropyl)adenine (81), and 1 -(3 -hydroxypropyl)uracil (82), with a view to determining the effect of the achiral residues on the c.d. spectra. 

The sequences flanking the initiator AUG in the SFV 26 S RNA are accommodated within the structure CAXXAUGa that has been considered a possible consensus sequence for a eukaryotic initiation site for translation (Kozak, 1981). Downstream from the initiation codon by 7 bases is a sequence of 11 nucleotides (AUCCCUACGCA), 9 of which (those underlined) are complementary to the purine-rich tract close to... 

There is one start codon (initiation codon), AUG, coding for methionine. Protein synthesis begins with methionine (Met) in eukaryotes, and formylmethionine (finet) in prokaryotes. 

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