A Ribosome Is a Ribonucleoprotein Particle 70S Made of a Small 30S and a Large 50S Subunit

IIe (α)
His (α 2)
Leu (α) Lys (α 2)
Met (α) Phe (α 2 β 2)
Trp (α 2) Ser (α 2)
Tyr (α 2) Pro (α 2)
Val (α) Thr(α 2)
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.2. Aminoacyl-Transfer RNA Synthetases Read the Genetic Code
Figure 29.14. Classes of Aminoacyl-tRNA Synthetases. Class I and class II synthetases recognize different faces of the
tRNA molecule. The CCA arm of tRNA adopts different conformations in complexes with the two classes of
synthetase.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a
Large (50S) Subunit
We turn now to ribosomes, the molecular machines that coordinate the interplay of charged tRNAs, mRNA, and proteins
that leads to protein synthesis. An E. coli ribosome is a ribonucleoprotein assembly with a mass of about 2700 kd, a
diameter of approximately 200 Å, and a sedimentation coefficient of 70S. The 20,000 ribosomes in a bacterial cell
constitute nearly a fourth of its mass.
A ribosome can be dissociated into a large subunit (50S) and a small subunit (30S) (Figure 29.15). These subunits can be
further split into their constituent proteins and RNAs. The 30S subunit contains 21 different proteins (referred to as S1
through S21) and a 16S RNA molecule. The 50S subunit contains 34 different proteins (L1 through L34) and two RNA
molecules, a 23S and a 5S species. A ribosome contains one copy of each RNA molecule, two copies of the L7 and L12
proteins, and one copy of each of the other proteins. The L7 protein is identical with L12 except that its amino terminus
is acetylated. Only one protein is common to both subunits: S20 is identical with L26. Both the 30S and the 50S subunits
can be reconstituted in vitro from their constituent proteins and RNA, as was first achieved by Masayasu Nomura in
1968. This reconstitution is an outstanding example of the principle that supramolecular complexes can form
spontaneously from their macromolecular constituents.
Electron microscopic studies of the ribosome at increasingly high resolution provided views of the overall structure and
revealed the positions of tRNA-binding sites. Astounding progress on the structure of the ribosome has been made by xray crystallographic methods, after the pioneering work by Ada Yonath. The structures of both the 30S and the 50S
subunits have been determined at or close to atomic resolution, and the elucidation of the structure of intact 70S
ribosomes at a similar resolution is following rapidly (Figure 29.16). The determination of this structure requires the
positioning of more than 100,000 atoms. The features of these structures are in remarkable agreement with
interpretations of less-direct experimental probes. These structures provide an invaluable framework for examining the
mechanism of protein synthesis.
29.3.1. Ribosomal RNAs (5S, 16S, and 23S rRNA) Play a Central Role in Protein
Synthesis
The prefix ribo in the name ribosome is apt, because RNA constitutes nearly two-thirds of the mass of these large
molecular assemblies. The three RNAs present 5S, 16S, and 23S are critical for ribosomal architecture and function.
They are formed by cleavage of primary 30S transcripts and further processing. The base-pairing patterns of these
molecules were deduced by comparing the nucleotide sequences of many species to detect conserved features, in
combination with chemical modification and digestion experiments (Figure 29.17). The striking finding is that ribosomal
RNAs (rRNAs) are folded into defined structures with many short duplex regions. This conclusion and essentially all
features of the secondary structure have been confirmed by the x-ray crystallographically determined structures.
For many years, ribosomal proteins were presumed to orchestrate protein synthesis and ribosomal RNAs were
presumed to serve primarily as structural scaffolding. The current view is almost the reverse. The discovery of
catalytic RNA made biochemists receptive to the possibility that RNA plays a much more active role in ribosomal
function. The detailed structures make it clear that the key sites in the ribosome are composed almost entirely of RNA.
Contributions from the proteins are minor. Many of the proteins have elongated structures that "snake" their way into the
RNA matrix (Figure 29.18). The almost inescapable conclusion is that the ribosome initially consisted only of RNA and
that the proteins were added later to fine tune its functional properties. This conclusion has the pleasing consequence of
dodging a "chicken and egg" question namely, How can complex proteins be synthesized if complex proteins are
required for protein synthesis?
29.3.2. Proteins Are Synthesized in the Amino-to-Carboxyl Direction
Before the mechanism of protein synthesis could be examined, several key facts had to be established. The results of
pulse-labeling studies by Howard Dintzis established that protein synthesis proceeds sequentially from the amino
terminus. Reticulocytes (young red blood cells) that were actively synthesizing hemoglobin were treated with [3H]
leucine. In a period of time shorter than that required to synthesize a complete chain, samples of hemoglobin were taken,
separated into α and β chains, and analyzed for the distribution of 3H within their sequences. In the earliest samples,
only regions near the carboxyl ends contained radioactivity. In later samples, radioactivity was present closer to the
amino terminus as well. This distribution is the one expected if the amino-terminal regions of some chains had already
been partly synthesized before the addition of the radioactive amino acid. Thus, protein synthesis begins at the amino
terminus and extends toward the carboxyl terminus.
29.3.3. Messenger RNA Is Translated in the 5 -to-3 Direction
The sequence of amino acids in a protein is translated from the nucleotide sequence in mRNA. In which direction is the
message read? The answer was established by using the synthetic polynucleotide
as the template in a cell-free protein-synthesizing system. AAA encodes lysine, whereas AAC encodes asparagine. The
polypeptide product was
Because asparagine was the carboxyl-terminal residue, we can conclude that the codon AAC was the last to be read.
Hence, the direction of translation is 5
3 .
The direction of translation has important consequences. Recall that transcription also occurs in the 5
3 direction
(Section 28.1.4). If the direction of translation were opposite that of transcription, only fully synthesized mRNA could be
translated. In contrast, because the directions are the same, mRNA can be translated while it is being synthesized. In
prokaryotes, almost no time is lost between transcription and translation. The 5 end of mRNA interacts with ribosomes
very soon after it is made, much before the 3 end of the mRNA molecule is finished. An important feature of
prokaryotic gene expression is that translation and transcription are closely coupled in space and time. Many ribosomes
can be translating an mRNA molecule simultaneously. This parallel synthesis markedly increases the efficiency of
mRNA translation. The group of ribosomes bound to an mRNA molecule is called a polyribosome or a polysome (Figure
29.19).
29.3.4. The Start Signal Is AUG (or GUG) Preceded by Several Bases That Pair with
16S rRNA
How does protein synthesis start? The simplest possibility would be for the first three nucleotides of each mRNA to
serve as the first codon; no special start signal would then be needed. However, the experimental fact is that translation
does not begin immediately at the 5 terminus of mRNA. Indeed, the first translated codon is nearly always more than 25
nucleotides away from the 5 end. Furthermore, in prokaryotes, many mRNA molecules are polycistronic, or
polygenic that is, they encode two or more polypeptide chains. For example, a single mRNA molecule about 7000
nucleotides long specifies five enzymes in the biosynthetic pathway for tryptophan in E. coli. Each of these five proteins
has its own start and stop signals on the mRNA. In fact, all known mRNA molecules contain signals that define the
beginning and end of each encoded polypeptide chain.
A clue to the mechanism of initiation was the finding that nearly half the amino-terminal residues of proteins in E. coli
are methionine. In fact, the initiating codon in mRNA is AUG (methionine) or, much less frequently, GUG (valine).
What additional signals are necessary to specify a translation start site? The first step toward answering this question was
the isolation of initiator regions from a number of mRNAs. This isolation was accomplished by using pancreatic
ribonuclease to digest mRNA-ribosome complexes (formed under conditions of chain initiation but not elongation). In
each case, a sequence of about 30 nucleotides was protected from digestion. As expected, each initiator region displays
an AUG (or GUG) codon (Figure 29.20). In addition, each initiator region contains a purine-rich sequence centered
about 10 nucleotides on the 5 side of the initiator codon.
The role of this purine-rich region, called the Shine-Dalgarno sequence, became evident when the sequence of 16S
rRNA was elucidated. The 3 end of this rRNA component of the 30S subunit contains a sequence of several bases that is
complementary to the purine-rich region in the initiator sites of mRNA. Mutagenesis of the CCUCC sequence near the 3
end of 16S rRNA to ACACA markedly interferes with the recognition of start sites in mRNA. This and other evidence
shows that the initiator region of mRNA binds to the 16S rRNA very near its 3 end. The number of base pairs linking
mRNA and 16S rRNA ranges from three to nine. Thus, two kinds of interactions determine where protein synthesis
starts: (1) the pairing of mRNA bases with the 3 end of 16S rRNA and (2) the pairing of the initiator codon on mRNA
with the anticodon of an initiator tRNA molecule.
29.3.5. Bacterial Protein Synthesis Is Initiated by Formylmethionyl Transfer RNA
The methionine residue found at the amino-terminal end of E. coli proteins is usually modified. In fact, protein synthesis
in bacteria starts with N-formylmethionine (fMet). A special tRNA brings formylmethionine to the ribosome to initiate
protein synthesis. This initiator tRNA (abbreviated as tRNAf) differs from the one that inserts methionine in internal
positions (abbreviated as tRNAm). The subscript "f" indicates that methionine attached to the initiator tRNA can be
formylated, whereas it cannot be formyl-ated when attached to tRNAm. In approximately one-half of E. coli proteins, Nformylmethionine is removed when the nascent chain is 10 amino acids long.
Methionine is linked to these two kinds of tRNAs by the same amino-acyl-tRNA synthetase. A specific enzyme then
formylates the amino group of methionine that is attached to tRNAf (Figure 29.21). The activated formyl donor in this
reaction is N 10-formyltetrahydrofolate (Section 24.2.6). It is significant that free methionine and methionyl-tRNAm are
not substrates for this transformylase.
29.3.6. Ribosomes Have Three tRNA-Binding Sites That Bridge the 30S and 50S
Subunits
A snapshot of a significant moment in protein synthesis was obtained by determining the structure of the 70S ribosome
bound to three tRNA molecules and a fragment of mRNA (Figure 29.22). As expected, the mRNA fragment is bound
within the 30S subunit. Each of the tRNA molecules bridges between the 30S and 50S subunits. At the 30S end, two of
the three tRNA molecules are bound to the mRNA fragment through anticodon-codon base pairs. These binding sites are
called the A site (for aminoacyl) and the P site (for peptidyl). The third tRNA molecule is bound to an adjacent site
called the E site (for exit).
The other end of each tRNA molecule interacts with the 50S subunit. The acceptor stems of the tRNA molecules
occupying the A site and the P site converge at a site where a peptide bond is formed. Further examination of this site
reveals that a tunnel connects this site to the back of the ribosome (Figure 29.23). The polypeptide chain passes through
this tunnel during synthesis.
29.3.7. The Growing Polypeptide Chain Is Transferred Between tRNAs on PeptideBond Formation
Protein synthesis begins with the interaction of the 30S subunit and mRNA through the Shine-Delgarno sequence. On
formation of this complex, the initiator tRNA charged with formylmethionine binds to the initiator AUG codon, and the
50S subunit binds to the 30S subunit to form the complete 70S ribosome. How does the polypeptide chain increase in
length (Figure 29.24)? The three sites in our snapshot of protein synthesis provide a clue. The initiator tRNA is bound in
the P site on the ribosome. A charged tRNA with an anticodon complementary to the codon in the A site then binds. The
stage is set for the formation of a peptide bond: the formylmethionine molecule linked to the initiator tRNA will be
transferred to the amino group of the amino acid in the A site. The transfer takes place in a ribosome site called the
peptidyl transferase center.
The amino group of the aminoacyl-tRNA in the A site is well positioned to attack the ester linkage between the initiator
tRNA and the formylmethionine molecule (Figure 29.25). The peptidyl transferase center includes bases that promote
this reaction by helping to form an -NH2 group on the A site aminoacyl-tRNA and by helping to stabilize the tetrahedral
intermediate that forms. This reaction is, in many ways, analogous to the reverse of the reaction catalyzed by serine
proteases such as chymotrypsin (Section 9.1.2). The peptidyl-tRNA is analogous to the acyl-enzyme form of a serine
protease. In a serine protease, the acyl-enzyme is generated with the use of the free energy associated with cleaving an
amide bond. In the ribosome, the free energy necessary to form the analogous species, an amino-acyl-tRNA, comes from
the ATP that is cleaved by the aminoacyl-tRNA synthetase before the arrival of the tRNA at the ribosome.
With the peptide bond formed, the peptide chain is now attached to the tRNA in the A site on the 30S subunit while a
change in the interaction with the 50S subunit has placed that tRNA and its peptide in the P site of the large subunit. The
tRNA in the P site of the 30S subunit is now uncharged. For translation to proceed, the mRNA must be moved (or
translo-cated) so that the codon for the next amino acid to be added is in the A site. This translocation takes place
through the action of a protein enzyme called elongation factor G (Section 29.4.3), driven by the hydrolysis of GTP. On
completion of this step, the peptidyl-tRNA is now fully in the P site, and the uncharged initiator tRNA is in the E site
and has been disengaged from the mRNA. On dissociation of the initiator tRNA, the ribosome has returned to its initial
state except that the peptide chain is attached to a different tRNA, the one corresponding to the first codon past the
initiating AUG. Note that the peptide chain remains in the P site on the 50S subunit throughout this cycle, presumably
growing into the tunnel. This cycle is repeated as new aminoacyl-tRNAs move into the A site, allowing the polypeptide
to be elongated indefinitely.
We can now understand why the amino terminus of the initial methionine molecule is modified by the attachment of a
formyl group. Chemical reactivity may have dictated this modification (Figure 29.26). Suppose that the amino-terminus
is not blocked. After the first peptidyl-transfer reaction, a dipeptide is linked to the tRNA in the P site. If a free amino
group is present in the terminal amino acid, this amino group can attack the carbonyl group of the ester linkage to the
tRNA, forming a very stable six-membered ring and terminating translation.
29.3.8. Only the Codon-Anticodon Interactions Determine the Amino Acid That Is
Incorporated
On the basis of the mechanism described in Section 29.3.7, 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
adding Raney nickel to Cys-tRNACys; the reaction removed the sulfur atom from the cysteine residue without affecting
its linkage to tRNA. Thus, a mischarged aminoacyl-tRNA was produced in which alanine was covalently attached to a
tRNA specific for cysteine.
Does this mischarged tRNA recognize the codon for cysteine or for alanine? The answer came when the tRNA was
added to a cell-free protein-synthesizing system. The template was a random copolymer of U and G in the ratio of 5:1,
which normally incorporates cysteine (encoded by UGU) but not alanine (encoded by GCN). However, alanine was
incorporated into a polypeptide when Ala-tRNACys was added to the incubation mixture. The same result was obtained
when mRNA for hemoglobin served as the template and [14C]alanyl-tRNACys was used as the mischarged aminoacyltRNA. The only radioactive tryptic peptide produced was one that normally contained cysteine but not alanine. Thus, the
amino acid in aminoacyl-tRNA does not play a role in selecting a codon.
In recent years, the ability of mischarged tRNAs to transfer their amino acid cargo to a growing polypeptide chain has
been used to synthesize peptides with amino acids not found in proteins incorporated into specific sites in a protein.
Aminoacyl-tRNAs are first linked to these unnatural amino acids by chemical methods. These mischarged aminoacyltRNAs are added to a cell-free protein-synthesizing system along with specially engineered mRNA that contains codons
corresponding to the anticodons of the mischarged aminoacyl-tRNAs in the desired positions. The proteins produced
have unnatural amino acids in the expected positions. More than 100 different unnatural amino acids have been
incorporated in this way. However, only l-amino acids can be used; apparently this stereochemistry is required for
peptide-bond formation to take place.
29.3.9. Some Transfer RNA Molecules Recognize More Than One Codon Because of
Wobble in Base-Pairing
What are the rules that govern the recognition of a codon by the anticodon of a tRNA? A simple hypothesis is that each
of the bases of the codon forms a Watson-Crick type of base pair with a complementary base on the anticodon. The
codon and anticodon would then be lined up in an antiparallel fashion. In the diagram in the margin, the prime denotes
the complementary base. Thus X and X would be either A and U (or U and A) or G and C (or C and G). According to
this model, a particular anticodon can recognize only one codon.
The facts are otherwise. As found experimentally, some pure tRNA molecules can recognize more than one codon. For
example, the yeast alanyl-tRNA binds to three codons: GCU, GCC, and GCA. The first two bases of these codons are
the same, whereas the third is different. Could it be that recognition of the third base of a codon is sometimes less
discriminating than recognition of the other two? The pattern of degeneracy of the genetic code indicates that this might
be so. XYU and XYC always encode the same amino acid; XYA and XYG usually do. Francis Crick surmised from
these data that the steric criteria might be less stringent for pairing of the third base than for the other two. Models of
various base pairs were built to determine which ones are similar to the standard A · U and G · C base pairs with regard
to the distance and angle between the glycosidic bonds. Inosine was included in this study because it appeared in several
anticodons. With the assumption of some steric freedom ("wobble") in the pairing of the third base of the codon, the
combinations shown in Table 29.3 seemed plausible.
The wobble hypothesis is now firmly established. The anticodons of tRNAs of known sequence bind to the codons
predicted by this hypothesis. For example, the anticodon of yeast alanyl-tRNA is IGC. This tRNA recognizes the codons
GCU, GCC, and GCA. Recall that, by convention, nucleotide sequences are written in the 5
3 direction unless
otherwise noted. Hence, I (the 5 base of this anticodon) pairs with U, C, or A (the 3 base of the codon), as predicted.
Two generalizations concerning the codon-anticodon interaction can be made:
1. The first two bases of a codon pair in the standard way. Recognition is precise. Hence, codons that differ in either of
their first two bases must be recognized by different tRNAs. For example, both UUA and CUA encode leucine but are
read by different tRNAs.
2. The first base of an anticodon determines whether a particular tRNA molecule reads one, two, or three kinds of
codons: C or A (one codon), U or G (two codons), or I (three codons). Thus, part of the degeneracy of the genetic code
arises from imprecision (wobble) in the pairing of the third base of the codon with the first base of the anticodon. We see
here a strong reason for the frequent appearance of inosine, one of the unusual nucleosides, in anticodons. Inosine
maximizes the number of codons that can be read by a particular tRNA molecule. The inosines in tRNA are formed by
deamination of adenosine after synthesis of the primary transcript.
Why is wobble tolerated in the third position of the codon but not in the first two? The 30S subunit has two adenine
bases (A1492 and A1493 in the 16S RNA) that form hydrogen bonds on the minor-groove side of the codon-anticodon
duplex. These interactions serve to check whether Watson-Crick base pairs are present in the first two positions of the
codon- anticodon duplex. No such inspection device is present for the third position so more-varied base pairs are
tolerated. This mechanism for ensuring fidelity is analogous to the minor-groove interactions utilized by DNA
polymerase for a similar purpose (Section 27.2.3). Thus, the ribosome plays an active role in decoding the codonanticodon interactions.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.15. Ribosomes at Low Resolution. Electron micrographs of (A) 30S subunits, (B) 50S subunits, and (C) 70S
ribosomes. [Courtesy of Dr. James Lake.]
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.16. The Ribosome at High Resolution. Detailed models of the ribosome based on the results of x-ray
crystallographic studies of the 70S ribosome and the 30S and 50S subunits. 23S RNA is shown in yellow, 5S RNA
in orange, 16S RNA in green, proteins of the 50S subunit in red, and proteins of the 30S subunit in blue.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.17. Ribosomal RNA Folding Pattern. (A) The secondary structure of 16S ribosomal RNA deduced from
sequence comparison and the results of chemical studies. (B) The tertiary structure of 16S RNA determined by xray crystallography. [Part A courtesy of Dr. Bryn Weiser and Dr. Harry Noller.]
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.18. Ribosomal Protein Structure. The structure of ribosomal protein L19 of the 50S ribosomal subunit
reveals a long segment of extended structure that fits through some of the cavities within the 23S RNA molecule.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.19. Polysomes. Transcription of a segment of DNA from E. coli generates mRNA molecules that are
immediately translated by multiple ribosomes. [From O. L. Miller, Jr., B. A. Hamkalo, and C. A. Thomas, Jr. Science 169
(1970):392.]
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.20. Initiation Sites. Sequences of mRNA initiation sites for protein synthesis in some bacterial and viral
mRNA molecules. Comparison of these sequences reveals some recurring features.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.21. Formylation of Methionyl-tRNA. Initiator tRNA (tRNAf) is first charged with methionine, and then a
formyl group is transferred to the methionyl- tRNAf from N 10-formyltetrahydrofolate.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.22. Transfer RNA-Binding Sites. (A) Three tRNA-binding sites are present on the 70S ribosome. They are
called the A (for aminoacyl), P (for peptidyl), and E (for exit) sites. Each tRNA molecule contacts both the 30S
and the 50S subunit. (B) The tRNA molecules in sites A and P are base paired with mRNA.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.23. Polypeptide Escape Path. A tunnel passes through the 50S subunit beginning at the site of peptide-bond
formation (shown in blue). The growing polypeptide chain passes through this tunnel.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.24. Mechanism of Protein Synthesis. The cycle begins with peptidyl-tRNA in the P site. An aminoacyltRNA binds in the A site. With both sites occupied, a new peptide bond is formed. The tRNAs and the mRNA are
translocated through the action of elongation factor G, which moves the deacylated tRNA to the E site. Once there, it is
free to dissociate to complete the cycle.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.25. Peptide-Bond Formation. The amino group of the aminoacyl-tRNA attacks the carbonyl group of the
ester linkage of the peptidyl-tRNA to form a tetrahedral intermediate. This intermediate collapses to form the peptide
bond and release the deacylated tRNA.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Figure 29.26. A Role for Formylation. With a free terminal amino group, dipeptidyl-tRNA can cyclize to cleave itself
from tRNA. Formylation of the amino terminus blocks this reaction.
III. Synthesizing the Molecules of Life
29. Protein Synthesis
29.3. A Ribosome Is a Ribonucleoprotein Particle (70S) Made of a Small (30S) and a Large (50S) Subunit
Table 29.3. Allowed pairings at the third base of the codon according to the wobble hypothesis
First base of anticodon Third base of codon
C
A
U
G
I
G
U
A or G
U or C
U, C, or A