Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 183

Before translation can begin, a specific amino acid is attached to each tRNA by its cognate aaRS (Figure 2.24). Each of these enzymes specifically recognizes only one amino acid and one class of tRNA—hence the name cognate. How each aaRS recognizes its own tRNA varies, but the anticodon (i.e., the three tRNA nucleotides that base pair with the complementary mRNA sequence [Figure 2.25]) is not the only determinant. Other variations in the tRNA structure, such as the identity of the discriminator base immediately upstream of the CCA at the 3′ end (Figure 2.17) and the size of the variable loop, also contribute to aaRS recognition specificity. In some cases, if the anticodon in a given tRNA is mutated, the cognate aaRS still attaches the amino acid for the original tRNA, and that amino acid is inserted at a different codon in the mRNA. This is the basis of nonsense suppression, which is discussed in chapter 3. The amino acid is attached to the terminal A residue on the tRNA, and the energy for the reaction is provided by cleavage of ATP. Finally, the tRNA with its amino acid is bound to the protein EF-Tu, which assists in delivery of the aa-tRNA to the ribosome.

Figure 2.23 Overview of translation. (A) The ribosomal A (aminoacyl) site is empty, the growing peptide chain is attached to the P (peptidyl) site tRNA, and the tRNA that previously contained the peptide chain is in the E (exit) site. (B) The tRNA bound to its amino acid and complexed with elongation factor Tu (EF-Tu) and guanosine triphosphate (GTP) comes into the empty A site and remains there if its anticodon matches the mRNA codon at that site; the E site tRNA leaves the ribosome. EF-Tu–GTP is converted to EF-Tu–GDP and released from the ribosome, then is recycled to EF-Tu–GTP by EF-Ts. (C) Peptidyltransferase (23S rRNA in the 50S ribosome) catalyzes peptide bond formation between the carboxyl end of the growing polypeptide carried by the P site tRNA and the amino end of the amino acid carried by the A site tRNA. (D) Translation elongation factor G (EF-G) catalyzes translocation of the A site tRNA to the P site, making room at the A site for another aminoacyl-tRNA. The previous P site tRNA, now stripped of its polypeptide, moves to the E site before exiting the ribosome.

Table 2.2 The genetic code

First position (5′ end)	Second position	Third position (3′ end)
U	C	A	G
U	Phe	Ser	Tyr	Cys	U
	Phe	Ser	Tyr	Cys	C
	Leu	Ser	Stop	Stop	A
	Leu	Ser	Stop	Trp	G
C	Leu	Pro	His	Arg	U
	Leu	Pro	His	Arg	C
	Leu	Pro	Gln	Arg	A
	Leu	Pro	Gln	Arg	G
A	Ile	Thr	Asn	Ser	U
	Ile	Thr	Asn	Ser	C
	Ile	Thr	Lys	Arg	A
	Met	Thr	Lys	Arg	G
G	Val	Ala	Asp	Gly	U
	Val	Ala	Asp	Gly	C
	Val	Ala	Glu	Gly	A
	Val	Ala	Glu	Gly	G

Figure 2.24 Aminoacylation of a tRNA by its cognate aminoacyl-tRNA synthetase (aaRS). ATP is used as a source of energy, and the amino acid is attached to the adenosine residue at the 3′ end of the tRNA. Each amino acid utilizes a dedicated aaRS.

Figure 2.25 Complementary pairing between a tRNA anticodon and an mRNA codon. The codon (green) is shown 5′-3′, and the tRNA is shown 3′-5′ to allow antiparallel pairing between the codon and the tRNA anticodon (orange).