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A2 Transfer RNA （ tRNA ）

A2 Transfer RNA （ tRNA ）. Role in translation Structure Synthesis and processing Modifications and nucleotides. 1. Role in translation. Transfer RNAs (tRNAs) are small molecules that act as adaptors during protein synthesis.

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A2 Transfer RNA （ tRNA ）

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  1. A2 Transfer RNA（tRNA）

  2. Role in translation • Structure • Synthesis and processing • Modifications and nucleotides

  3. 1. Role in translation • Transfer RNAs (tRNAs) are small molecules that act as adaptors during protein synthesis. • nucleotide sequence of mRNA---tRNA---amino acid sequence of the polypeptide • Each tRNA bind a specific amino acid, recognize its codon in mRNA, place this amino acid in the growing polypeptide.

  4. 3’ 3’ ACC ACC 5’ 5’

  5. 2. tRNA structure • Primary structure: 74-95nt, with modified nucleotides • Secondary structure: cloverleaf['kləuvəli:f] composed of a series of stem-loop structure known as arms. • acceptor arm （氨基酸臂）: CCA as the attachment site for amino acid • D or DHU arm (二氢尿嘧啶环）: dihydrouracil • anticodon arm （反密码子环）: recognize and bind codon in the mRNA

  6. optional arm or extra or variable arm: Class I tRNA, 2-3 nucleotides; Class ⅡtRNA, 13-21 nucleotides, up to five base pairs • TψC arm: contains Tψ • Invariant, semi-variant and not conserved nucleotides • 3D-structure(tertiary structure): acceptor arm and anticodon loop are at opposite ends.

  7. 3. Synthesis and processing • Synthesis: RNA polymerase Ⅲ transcribe tRNA genes which exist as multiple copies • Processing: RNase P and RNase D • Several tRNA genes may be transcribed together as a single pre-tRNA which is then processed by ribonucleases that cleave at the 5’ and 3’ ends of each tRNA sequence.

  8. In prokaryotes, processing is carried out by a series order of steps by ribonucleases (RNase D and RNase P) and RNA component of RNase P has catalytic activity known as ribozyme. • CCA (the attachment site for amino acid) is present in the coding sequence although sometimes is removed by RNase D.

  9. In eukaryotes, introns in pre-tRNA transcript must be removed during processing and CCA is not present in eukaryotic tRNA gene and is added later.

  10. 4.Modification of nucleotides： tRNA contains modified bases that influence its pairing properties • Methylation of the ribose sugar of the nucleotide. e.g: 7-methylguanosine. • Base rearrangements: atom position interchanging. e.g: uradine to pseudouradine • Double-bond saturation: uradine to dihydrouridine • Deamination: guanosine to inosine • Sulfur substitution: uradine to 4-thiouradine (硫尿核苷） • Addition of larger groups: guanosine to queosine

  11. All of the four bases in tRNA can be modified.

  12. Key points • The role of tRNA in translation • Processing and modifications of tRNA

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