Page 131 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

290 Chapter 8 Gene Expression: The Flow of Information from DNA to RNA to Protein

Figure 8.18 tRNA structure. The nucleotide sequence of a Figure 8.19 Aminoacyl-tRNA synthetases attach tRNAs
tRNA (the primary structure) folds to form characteristic secondary to their corresponding amino acids. The aminoacyl-tRNA
and tertiary structures. The anticodon and the amino acid attachment synthetase has recognition sites for an amino acid, the corresponding
site are at opposite ends of the L-shaped tertiary structure. Several tRNA, and ATP. The synthetase first activates the amino acid,
unusual bases of the tRNA, indicated as I, ψ, UH 2 , mI, m 2 G, and mG, forming an AMP-amino acid. The enzyme then transfers the amino
are enzymatically modified variants of A, G, C, and U. acid’s carboxyl group from AMP to the hydroxyl (–OH) group of the
Primary 5' 3' ribose at the 3′ end of the tRNA, producing a charged tRNA.
structure G G G C G U G U … … U C C A C C A
Gly tRNA synthetase
3'
Secondary OH Tertiary O -
–
-
structure structure H O –P=O H
A Amino acid — O – —
–
C attachment 5' H—C—NH 2 O –P=O H—C—NH 2
-
—
C – — O -
Yeast A site O = C — O - - O – O = C — O–P O
–
tRNA Ala C … G 5' 3' Amino O –P=O – =
–
O
C … G acid Adenosine O — Adenosine
–
OH (AMP)
U … G Amino acid (Gly) OH (ATP)
G … C attachment
C … G tRNA GIy
U U site
C … G
G A U U C C G G A U
…
…
…
C T G G C C U … mG … G C G U … A G UH 2 C
…
…
G UH 2 C m 2 G C G C G A U H 2 G G
G A G … C
A … U H
G … C —
H—C—NH 2
G … C —
G … C O — C = O
3' ml C … G I U U 5' Charged tRNA GIy
mRNA … …
G C A
5' Codon 3' Anticodon
for Ala
At one end of the L, the tRNA carries an anticodon: acid. Figure 8.19 shows the two­step process that estab­
three nucleotides complementary to an mRNA codon spec­ lishes the covalent bond between an amino acid and the 3′
ifying the amino acid carried by the tRNA (Fig. 8.18). The end of its corresponding tRNA. A tRNA covalently cou­
anticodon never forms base pairs with other regions of the pled to its amino acid is called a charged tRNA. The bond
tRNA; it is always available for base pairing with its com­ between the amino acid and tRNA contains substantial en­
plementary mRNA codon. As with other complementary ergy that is later used to drive peptide bond formation.
base sequences, during pairing at the ribosome, the strands
of anticodon and codon run antiparallel to each other. If, for The crucial role of base pairing between
example, the anticodon is 3′ CCU 5′, the complementary codon and anticodon
mRNA codon is 5′ GGA 3′, specifying the amino acid While attachment of the appropriate amino acid charges a
glycine. At the other end of the L, where the 5′ and 3′ ends tRNA, the amino acid itself does not play a significant role
of the tRNA strand are found (Fig. 8.18), the appropriate in determining where it becomes incorporated in a growing
amino acid is attached to the tRNA’s 3′ end. polypeptide chain. Instead, the specific interaction between a
tRNA’s anticodon and an mRNA’s codon makes that deci­
Aminoacyl-tRNA synthetases: The molecular sion. A simple experiment illustrates this point (Fig. 8.20).
translators of the genetic code Researchers can subject a charged tRNA to chemical treat­
Enzymes known as aminoacyl-tRNA synthetases connect ments that, without altering the structure of the tRNA,
change the amino acid it carries. One treatment replaces the
the tRNA to the amino acid that corresponds to the antico­ cysteine carried by tRNA with alanine. When investiga­
Cys
don. These enzymes are extraordinarily specific, recogniz­ tors then add the tRNA charged with alanine to a cell­free
Cys
ing unique features of a particular tRNA including the translational system, the system incorporates alanine into the
anticodon, while also recognizing the corresponding amino growing polypeptide wherever the mRNA contains a cyste­
acid (see the opening figure of this chapter). ine codon complementary to the anticodon of the tRNA Cys.
Aminoacyl­tRNA synthetases are, in fact, the only
molecules that read the languages of both nucleic acid and
protein. Normally, one aminoacyl­tRNA synthetase exists Wobble: One tRNA, more than one codon
for each of the 20 common amino acids. Each synthetase Although at least one kind of tRNA exists for each of the 20
functions with only one amino acid, but the enzyme may common amino acids, cells do not necessarily carry tRNAs
recognize several different tRNAs specific for that amino with anticodons complementary to all of the 61 possible