In some mRNAs, the third base upstream of the start codon is purine and the first base downstream is guanine. Interacts with the initiating tRNA.

The poly-A tail at the 3' end can enhance the recruitment of key translation initiation factors and improve translation.

Primary structure: All tRNAs end with a 5'-CCA-3' sequence at the 3' end, which is the site where the tRNA and associated amino acids are bound by aminoacyl-tRNA synthetase

Secondary structure: "Clover type" one receptor arm, three stem loops (ΨU loop, D loop (dihydrouracil), anticodon loop), variable loop

Tertiary structure: L-shaped, maintaining force: ① hydrogen bonding, ② interaction between bases and phosphate skeleton, ③ base stacking effect

The first step is adenylation of amino acids: the amino acids react with ATP and are acylated by adenylate to form aminoacyl-adenylate, while releasing pyrophosphate.

The second step is tRNA loading: the aminoacyl-adenylate (still tightly bound to the aminoacyl-tRNA synthetase) reacts with the tRNA, releasing AMP.

Receptor Arm - Determinant

anticodon loop

Ribosomes are unable to recognize their specificity

Large subunit: Contains the peptidyl transferase center, responsible for the formation of peptide bonds; the channel through which the polypeptide chain leaves.

Small subunit: contains the decoding center, responsible for reading; mRNA entry and exit channel

Ribosomes must be recruited to the mRNA

Loading tRNA must be placed in the P site of the ribosome

The ribosome must be positioned precisely on the start codon (critical)

Many prokaryotic cells do not start with Met: deformylase removes the formyl group, and aminopeptidase cleaves Met and one or two other amino acids at the amino terminus.

Process (starting from small subunit)

The start codon is different from the start aminoacyl tRNA: prokaryotes: AUG, GUG, UUG, fMet~; eukaryotes: AUG, Met-tRNA [Met]

Different from ribosome pairing: Prokaryotic mRNA has an SD sequence that can pair with 16SRNA, but eukaryotes do not.

Different initial recognition mechanisms

The mechanisms of formation of the initiation complex are different: the binding of eukaryotic initiating tRNA to the small subunit always occurs before binding to the mRNA. Two GTP-binding proteins place the initiating aminoacyl-tRNA at the future P site of the small subunit to form a 43S pre-initiation complex. Various initiation factors are subsequently added to form a 48S pre-initiation complex.

Different starting factors: 3 in prokaryotes and more than a dozen in eukaryotes

Different energy consumption: prokaryotes do not need to consume ATP to unravel the secondary structure during the initial process, while eukaryotes need to consume ATP.

First, under the guidance of the codon at the A site, the correct aminoacyl tRNA is placed at the A site

Second, the aminoacyl tRNA at A site forms a peptide bond with the peptide chain on the peptidyl acyl tRNA at P site, and the polypeptide chain moves from the P site to the A site.

Third, the formed peptidyl tRNA at the A site and the corresponding codon must be translocated to the P site to prepare the ribosome for the next cycle of codon recognition and peptide bond formation.

Function: Bind and hydrolyze GTP. Only when EF-Tu binds to GTP, EF-Tu can bind to aminoacyl tRNA. After GTP is hydrolyzed, the aminoacyl tRNA is released.

Conditions of action: Only when tRNA is placed in the A site and the correct codon-anticodon pairing can EF-Tu interact with the factor binding center

The two linked adenine residues of the small subunit 16S rRNA form a tight interaction with the minor groove formed by each correct pairing between the anticodon and the first two bases of the codon.

The second mechanism that helps ensure correct anticodon-codon pairing involves the GTPase activity of EF-Tu. Mispairing of even one base will cause a sharp decrease in the GTPase activity of EF-Tu.

The third mechanism to ensure the correctness of base pairing is a correction mechanism after the release of EF-Tu. In order to successfully carry out the peptidyl transferase reaction, the aminoacyl-tRNA must rotate into the peptidyl transferase center of the large subunit, that is, become in place.

Once the peptidyl transferase reaction begins, the tRNA at site P is deacetylated and the polypeptide chain is attached to the tRNA at site A.

The initial step of translocation is coupled to the peptidyl transferase reaction. Once the polypeptide chain moves to the tRNA at the A site, the 3' end of the tRNA moves to the P site of the large subunit, and the deaminated P The site tRNA is located at the E site of the large subunit and no longer binds to the polypeptide.

The completion of translocation requires the action of EF-G elongation factor. When EF-G-GTP is bound, it interacts with the factor binding center of the large subunit to stimulate GTP hydrolysis. GTP hydrolysis changes the EF-G-GTP conformation, allowing it to enter the small subunit and stimulate A-site tRNA translocation

Together these events result in translocation of the A site tRNA to the P site, movement of the P site to the E site, and movement of the mRNA by three amino acids

Class I release factor

Class II release factor

After the release of class I RF-stimulated peptides, the conformational changes of ribosomes and class I factors induce RF3 to exchange GDP and bind GTP. The binding of RF3 to GTP leads to the formation of high-affinity interactions with ribosomes, replacing class I molecules. Binding to ribosomes, RF3-GDP has weak affinity to ribosomes and is quickly released.

tmRNA: A chimeric RNA molecule that is part tRNA and part mRNA

SsrA RNA: a kind of tmRNA, the 3' region is similar to tRNA, can load Ala and bind EF-Tu-GTP; it is huge and cannot bind to the A site during normal elongation.

Mechanism: When a ribosome stalls at the 3' end of the mRNA, SsrA Ala-EF-TU-GTP binds to the A site of the ribosome and participates in the peptidyl transferase reaction. The defective mRNA is released from the ribosome and is quickly The proteolytic enzyme is degraded and the ribosome re-enters the translation cycle.

Nonsense codon-mediated mRNA decay

No stop codon-mediated decay

terminal termination mediated decay

Commonality: They all need to detect defective mRNA and degrade it through the translation process of damaged mRNA. They rely on the mechanism of translation and do not directly affect it.