Omar Bennett
The initiation of translation is the first step in protein synthesis, where the ribosome recognizes the mRNA start codon and sets the reading frame. The mechanisms and molecular machinery for translation initiation differ between prokaryotes and eukaryotes. Prokaryotic translation initiation is relatively simple and generally involves the interaction between the Shine-Dalgarno (SD) sequence in the mRNA and the anti-SD sequence in the ribosomal RNA, forming a pre-initiation complex. On the other hand, eukaryotic translation initiation is more complex, requiring multiple initiation factors and relying on protein-RNA and protein-protein interactions. The eukaryotic initiation factor-2 (eIF-2) plays a crucial role in this process, binding to the small ribosomal subunit and initiating the assembly of the initiation complex.
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What You'll Learn
- Prokaryotic translation initiation involves the Shine-Dalgarno (SD) sequence in the 5′ UTR of an mRNA
- Eukaryotic translation initiation involves a scanning mechanism
- Eukaryotic initiation requires at least 11 factors
- Prokaryotic initiation involves direct interaction of ribosomal RNA with mRNA
- Prokaryotic initiation involves the 30S initiation complex
Prokaryotic translation initiation involves the Shine-Dalgarno (SD) sequence in the 5′ UTR of an mRNA
The SD sequence was proposed by Australian scientists John Shine and Lynn Dalgarno in 1973. They identified the nucleotide tract at the 3' end of E. coli 16S ribosomal RNA (rRNA) as pyrimidine-rich, with the specific sequence ACCUCCUUA. They suggested that these ribosomal nucleotides recognize the complementary purine-rich sequence AGGAGGU, found upstream of the start codon AUG in various mRNAs present in viruses that affect E. coli.
The base pairing between the Shine-Dalgarno sequence in mRNA and the 3' end of 16S rRNA is crucial for the initiation of translation by bacterial ribosomes. This complementary relationship between rRNA and the Shine-Dalgarno sequence in mRNA indicates that the sequence at the 3'-end of the rRNA determines the ability of the prokaryotic ribosome to translate a specific gene in an mRNA. The core portions of the SD sequence (GAGG, GGAG, and AGGA) were identified in the appropriate position preceding 35 out of 72 initiation sites evaluated.
The interaction between the SD and anti-SD sequences, known as the SD interaction, enhances initiation by anchoring the small (30S) ribosomal subunit around the initiation codon, forming a preinitiation complex. This interaction is considered a universal mechanism of translation initiation in prokaryotes. Mutations in the Shine-Dalgarno sequence can impact translation in prokaryotes, increasing or decreasing mRNA-ribosome pairing efficiency.
In eukaryotes, translation initiation is more complex and primarily relies on protein-RNA and protein-protein interactions. It involves the recruitment of a ribosome-initiator tRNA complex to the initiation codon of a messenger RNA. The small (40S) ribosomal subunit binds to the 7-methyl guanosine cap at the 5′ end of an mRNA and moves along the mRNA until it encounters an AUG codon.
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Eukaryotic translation initiation involves a scanning mechanism
The initiation of protein synthesis involves the recruitment of a ribosome·initiator tRNA complex to the initiation codon of a messenger RNA. While prokaryotic translation initiation involves the direct interaction of the ribosomal RNA with the mRNA, eukaryotic translation initiation involves a scanning mechanism.
The scanning mechanism in eukaryotic translation initiation involves the small (40S) ribosomal subunit, with several initiation factors, binding to the 5' end of an mRNA. The 40S ribosomal subunit binds to the initiator tRNA (tRNAi). This initiator tRNA is charged with methionine in eukaryotes and archaea and with N-formyl-methionine in bacteria. The small ribosomal subunit then scans along the mRNA strand until it reaches the start codon AUG. The anticodon on the tRNAi then binds to the start codon via base pairing.
The 43S pre-initiation complex, accompanied by protein factors, moves along the mRNA chain toward its 3′-end in a process known as 'scanning'. This process helps the complex reach the start codon, which is typically AUG. The recognition of the AUG codon is facilitated by the presence of a particular sequence, such as the Kozak sequence. Once the 43S complex is at the initiation AUG, the tRNAi-Met is positioned over the AUG.
The scanning mechanism in eukaryotic translation initiation is a complex process involving multiple factors and interactions. The process begins with the formation of the 43S pre-initiation complex, which includes the Met-tRNAiMet delivered by eIF2 to the P site of the 40S ribosomal subunit. The 43S complex is then recruited to the 5' end of the mRNA by eIF3 and the eIF4 factors. The scanning of the 5' untranslated region (UTR) and recognition of the AUG codon are crucial steps in this process.
The exact molecular mechanism of the scanning process remains unclear. However, it is known that eIF1 plays a crucial role in the positioning of the 43S complex at the initiation codon. The function of eIF1 could be to participate in scanning, destabilize incorrectly positioned complexes, or stabilize correctly positioned ones.
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Eukaryotic initiation requires at least 11 factors
The initiation of protein synthesis involves the recruitment of a ribosome-initiator tRNA complex to the initiation codon of a messenger RNA. In prokaryotes, this involves the direct interaction of ribosomal RNA with mRNA. In contrast, eukaryotes have evolved a more complex mechanism that relies on protein-RNA and protein-protein interactions. This complexity allows for enhanced regulation, and many new mechanisms have been adopted by eukaryotes to control protein synthesis.
The multifactor complex is an intermediate that facilitates the Met-tRNAiMet recruitment to the 40S ribosomal subunit. The 43S complex is formed when the GTP-eIF-2/40S ribosome complex associates with the initiator tRNA charged with methionine (Met-tRNAi). Once bound to the mRNA’s 5′ m7G cap, the 43S complex starts travelling down the mRNA until it reaches the initiation AUG codon. The tRNAi-Met is then positioned over the AUG.
The largest and most complex initiation factor, eIF3, is composed of up to 11 nonidentical subunits in mammals. It is thought to promote the dissociation of 40S and 60S ribosomal subunits and is required for mRNA recruitment to the preinitiation complex. eIF3 also prevents 60S subunits from disrupting the 43S preinitiation complex. eIF3, eIF1, and eIF1A are recruited to 40S subunits during recycling, while eIF2–GTP–Met-tRNAMeti attaches to recycled 40S subunits to form 43S complexes.
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Prokaryotic initiation involves direct interaction of ribosomal RNA with mRNA
Prokaryotic initiation involves the direct interaction of ribosomal RNA with mRNA. This is in contrast to eukaryotes, which have evolved a more complex mechanism that relies on protein-RNA and protein-protein interactions.
In prokaryotes, the interaction between the Shine-Dalgarno (SD) sequence in the 5′ UTR of an mRNA and the anti-SD sequence in the 3′ end of a 16S ribosomal RNA initiates translation. This interaction anchors the small (30S) ribosomal subunit around the initiation codon to form a preinitiation complex. The SD interaction is considered a universal mechanism of translation initiation in prokaryotes, as alterations to the SD or anti-SD sequence strongly inhibit protein synthesis.
There are, however, two exceptional mechanisms of prokaryotic translation initiation that do not require the SD sequence. One of these mechanisms is mediated by a ribosomal protein S1 (RPS1), a component of the 30S ribosomal subunit. The other mechanism involves leaderless mRNA, which lacks the 5′ UTR region.
In eukaryotes, the process is more complex, and at least eleven different initiation factors are required to properly initiate translation. The first step involves the formation of the 43S pre-initiation complex, where the methionyl-initiator tRNA (Met-tRNAiMet) is delivered by eIF2 to the P site of the 40S ribosomal subunit. The 43S complex is then recruited to the 5' end of the mRNA by eIF3 and eIF4 factors. The complex moves along the mRNA until it encounters an AUG codon, which indicates the start of the mRNA's reading frame. Once the initiation codon is reached, the tRNAi-Met is positioned over the AUG, and translation can begin.
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Prokaryotic initiation involves the 30S initiation complex
The formation of the 30S IC is a critical checkpoint in regulating gene expression. It requires the presence of initiation factors IF1, IF2, and IF3, with IF3 and IF1 controlling the fidelity of the process while IF2 recruits the initiator tRNA. Cryo-electron microscopy studies have provided valuable insights into the structure of the 30S IC, revealing the positions of the tRNA and initiation factors. The binding of initiation factors and tRNA induces a rotation of the head relative to the body of the 30S subunit, which is important for the subsequent joining with the 50S subunit to form a functional ribosome.
The initiation phase of protein synthesis in prokaryotes involves the assembly of the 30S IC, which includes the three initiation factors (IF1, IF2, and IF3), mRNA, and initiator tRNA (fMet-tRNA). During this phase, proper codon-anticodon base pairing has not yet occurred. The recognition of the actual mRNA start codon progresses through several checkpoints, with the first being a conformational change in the 30S IC that allows for codon-anticodon base pairing in the P-site, resulting in a 'locked' 30S IC.
The 'locked' 30S IC is structurally competent for joining with the large ribosomal subunit (50S), leading to the formation of the 70S ribosomal initiation complex (70SIC). During this transition, IF1 and IF3 dissociate from the ribosome, while IF2 remains bound until it delivers the acceptor end of fMet-tRNA, facilitating the formation of the first peptide bond. The 30S IC plays a crucial role in ensuring accurate and efficient translation initiation in prokaryotes, setting the stage for successful protein synthesis.
Additionally, it is worth noting that while the 30S IC is a key feature of prokaryotic initiation, eukaryotic initiation involves a more complex machinery. Eukaryotes have evolved a sophisticated mechanism that relies on protein-RNA and protein-protein interactions, utilizing the unique mRNA structures such as the 5' cap and the poly (A) tail. In eukaryotes, the initiation process typically involves the small 40S ribosomal subunit and multiple initiation factors, including eIF1, eIF2, eIF3, and eIF4.
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Frequently asked questions
A translation initiation complex is a complex of molecules that come together to begin the process of translation, which is one stage of protein synthesis.
Prokaryotic translation initiation involves the interaction of the Shine-Dalgarno (SD) sequence in the 5′ UTR of an mRNA with the anti-SD sequence in the 3′ end of a 16S ribosomal RNA. This anchors the small (30S) ribosomal subunit around the initiation codon. There are two exceptions to this mechanism: one involves a ribosomal protein called RPS1, and the other uses leaderless mRNA that lacks its 5′ UTR.
Eukaryotic translation initiation is a complex process that involves the assembly of an 80S initiation complex on an mRNA. It requires at least 11-12 initiation factors, including eIFs 1, 1A, 3, 4, and 5, to ensure that the methionyl-initiator tRNA (Met-tRNAiMet) is brought to the P site of the ribosome. The small (40S) ribosomal subunit binds to the 5' end of an mRNA and moves along until it encounters an AUG codon.
Prokaryotic translation initiation is relatively simple, while eukaryotic translation initiation is more complex and requires more components. The mechanism of translation initiation in prokaryotes has remained largely unchanged during evolution, while eukaryotes have evolved a more sophisticated mechanism that relies on protein-RNA and protein-protein interactions.
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