Cognate tRNA-mRNA

T. thermophilus 30S ribosome Cognate tRNA-mRNA IIBM 3.31 1... 

T. thermophilus 30S ribosome Near-cognate tRNA-mRNA Paromomycin IFJG 3.00 13... 

In order that tRNA molecules can function as adaptors between amino acids and mRNA, two things must be true. First, there must be a mechanism for attaching each amino acid specifically to its cognate tRNA. Doing so will create molecules known as aminoacyl-tRNAs. Secondly, each aminoacyl-tRNA must recognize the correct triplet code for that amino acid on mRNA. 1 am going to worry about the former issue first. 

Now that we have specifically attached each amino acid to its cognate tRNA, we need to make the interface between each aminoacyl-tRNA and the correct triplet codon on mRNA. The aminoacyl moiety has nothing to do with this it is simply a matter of codon-anticodon recognition, which is shown schematically in figure 13.3. 

On the basis of the mechanism described on page 871, the base-pairing interaction between the anticodon on the incoming tRNA and the codon in the A site on mRNA determines which amino acid is added to the polypeptide chain. Does the amino acid attached to the tRNA play any role in this process This question was answered in the following way. First, cysteine was attached to its cognate tRNA. The attached cysteine unit was then converted into alanine by removing the sulfor atom from the side chain in cysteine in a reaction catalyzed by Raney nickel the reaction removed... 

Figure 18.13 shows the complete structure of the Themtus thermophilus 70s ribosome at 5.5 A resolution in the presence of mRNA and cognate tRNAs bound in the A (aminoacyl), P (peptidyl), and E (exit) sites, fiewed from the top, with the... 

Protein biosynthesis occurs in ribosomes, the cellular assembly factory where amino acids are covalently linked into a polypeptide chain along the template mRNA. The amino acids are transported into the ribosome in an activated state via coupling to specific carriers known as transfer RNAs (tRNA). The selection of specific amino acids from the cellular pool of amino acids (Tables 1.2 and 1.3) and the task of charging to cognate tRNAs are performed by specific aminoacyl-... 

Figure 3 Cognate and near-cognate codon-anticodon interactions. The anticodon ioop of tRNA is shown as an example interacting with various codons on the mRNA. In correct, cognate codon-anticodon pairings, two Watson-Crick base pairs can be formed in the first two positions while the third position contains either a Watson-Crick or a wobble base pair.

Each tRNA has an anticodon sequence that allows it to pair with the codon for its cognate amino acid in the mRNA. Because base pairing is involved, the orientation of this interaction wEl be complementary and antiparallel as shown in Figure T4-6. The arg-tRNA 8 has an anticodon sequence, UCG, allowing it to pair with the arginine codon CGA. 

The second phase of protein synthesis is the elongation cycle. This phase begins with the insertion of an aminoacyl-lRKA into the empty A site on the ribosome. The particular species inserted depends on the mRNA codon in the A site. The cognate ami noacyl-tKN A does not simply leave the synthetase and diffuse to the A site. Rather, it is delivered to the A site in association with a 4. Tkd protein called elongation factor Tu (EF-Tu). Elongation factor Tu, another member of the G-protein family, requires GTP to bind aminoacyl-tRNA (Figure 30.23) and to bind the ribosome. The binding of... 

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