11 - Translation
A. Introduction to Translation
Translation is a cellular process that involves the translation of the language of nucleic acids (A, U, G, C) contained within the mRNA molecule into the language of amino acids (met, ala, his, etc.) contained in a synthesized protein. This translation process relies on a code that converts nucleic acid sequence into amino acid sequence (genetic code). Note that the genetic code that is translated into protein functions within the mRNA molecule, not in the DNA.
The mRNA is read by the ribosome during translation as a group of three nucleotides (triplet code). Each group of three nucleotides within the mRNA sequence is called a codon.
The mRNA contains several sequence features that play a role in translation (figure 11.1). These sequence features are listed from 5’ to 3’ along the mRNA molecule, as follows:
- 5’ untranslated region (5’-UTR). The 5’-UTR of the mRNA is not translated by the ribosome. Instead, the 5’-UTR allows the ribosome to bind to the mRNA and positions the ribosome to interact properly with the start codon (see below). In bacteria, the 5’-UTR of the mRNA contains a specific nucleotide sequence called the Shine-Dalgarno sequence that serves as the binding site for the ribosome. In eukaryotes, the 5’-UTR contains the 7-methylguanosine (7-mG) cap, which serves as the ribosome binding site, and a specific nucleotide consensus sequence called the Kozak sequence, which positions the ribosome to recognize the start codon.
- Start codon. The start codon in the mRNA codes for the first amino acid in the synthesized protein. The start codon is usually 5'-AUG-3’ and encodes the amino acid N-formylmethionine (fmet) in bacteria and methionine (met) in eukaryotes.
- Sense codons. After the start codon, the mRNA is read as consecutive three nucleotide long sense codons. Each sense codon specifies an amino acid in the protein. Note that the start codon and the sense codons constitute the coding region of a gene.
- Stop codon. The stop codon in the mRNA serves as a signal to end the process of translation. The possible stop codons within prokaryotic and eukaryotic mRNAs include 5’-UAG-3’, 5’-UAA-3’, or 5’-UGA-3’. The stop codon does not encode an amino acid within the synthesized polypeptide.
- 3’ untranslated region (3’-UTR). The 3’-UTR of the mRNA, located downstream of the stop codon, plays a role in transcriptional termination. In prokaryotes, the 3’-UTR contains the rut, stem-loop nucleotides, and the uracil-rich sequences that play a role in rho (ρ)-dependent and rho (ρ)-independent transcriptional termination. In eukaryotes, the 3’-UTR contains the polyadenylation signal sequences recognized by the endonuclease CPSF and CstF during transcription termination.
- Identify one important sequence feature found in the 5’-UTR in bacteria.
- Identify two important sequence features found within the 5’-UTR of eukaryotes.
- What amino acid is encoded by 5’-AUG-3’ in bacteria?
- What amino acid is encoded by 5’-AUG-3’ in eukaryotes?
- What is the function of the coding region?
- What is the function of the stop codon?
- Does the stop codon encode an amino acid?
- What is the function of the 3’-UTR?
The Genetic Code
There are 20 amino acids typically incorporated in proteins; however, there are 64 possible combinations of four nucleotides (A, U, G, and C) in a triplet codon. As a result, there are more codon than amino acid possibilities (see figure 11.2). The genetic code is degenerate, meaning that more than one codon can encode a particular amino acid type. For example, the amino acid valine (val) is encoded by four codons (5’- GUU-3’, 5’-GUC-3’, 5’-GUA-3’, and 5’-GUG-3’) that differ only in the 3’-most base (wobble base). These four valine codons are called synonymous codons.
The genetic code is also unambiguous, meaning that a particular codon sequence only encodes one type of amino acid. For example, the codon 5’-AAA-3’ always encodes the amino acid lysine (Lys). Further, the genetic code is commaless, meaning that codons are read by the ribosome consecutively one after another as triplets, with no spacer nucleotides between the codons. The codons are also nonoverlapping, meaning each nucleotide in the coding region of the mRNA is a member of only one codon. Finally, the same genetic code is used in nearly all organisms (in other words, the code is universal).
- How would you use the words degenerate, unambiguous, commaless, nonoverlapping, and universal to describe the genetic code?
Directionality of Polypeptides
The polypeptide chains that are synthesized during translation have directionality. The amino acid encoded by the start codon (closer to the 5’ end of the mRNA) contains a free amino (NH3+) group and thus is said to be the amino or N-terminus of the polypeptide.
As the polypeptide is synthesized, a peptide bond is formed between the carboxyl group (COO-) of the growing amino acid chain and the amino group of the incoming amino acid (see figure 11.3). Peptide bond formation involves a condensation reaction that releases water as a product.
The final amino acid added to a polypeptide contains a free carboxyl group and is called the C-terminal or carboxyl terminal end. This end of the polypeptide corresponds to a codon closer to the 3’ end of the mRNA.
- How do the N- and C-terminal ends of a protein correlate with the 5' and 3' ends of an mRNA molecule?
The Adaptor Hypothesis
Transfer RNA molecules (tRNAs) act as adaptor molecules in translation. tRNAs function to:
- Recognize the triplet nucleotide sequence of the codon within the mRNA. This recognition process involves hydrogen bonding between the mRNA codon and an anticodon sequence within the tRNA.
- Carry a specific amino acid. tRNA molecules carry amino acids to the ribosome. The carried amino acid is then incorporated into the growing polypeptide. The amino acid carried by the tRNA is specified by the anticodon sequence.
There are many different types of tRNA molecules in a cell. Each type is encoded by a different gene and has a unique nucleotide sequence. tRNA molecules are named according to the amino acid they carry. For example, tRNAval carries the amino acid valine, while tRNAphe carries the amino acid phenylalanine.
- What are the two functions of a tRNA molecule?
Features of tRNAs
All types of tRNA molecules have similar structural features (figure 11.4):
- tRNA molecules are small. Each tRNA is 75–90 nucleotides in length.
- tRNA molecules are shaped like a cloverleaf. Three stem-loop structures are formed by hydrogen bonding within the tRNA molecule. The anticodon is located within one of these stem-loop structures, called the anticodon loop.
- tRNA molecules contain a 3’ single-stranded region called an acceptor stem. The acceptor stem region is covalently linked to an amino acid.
- tRNA molecules contain variable regions. The tRNA variable regions differ between tRNA types (for example, tRNAphe versus tRNAval) in the number of nucleotides and the specific nucleotide sequence that they contain. Variability in these regions can make the stem loops different sizes.
- tRNA molecules contain unusual bases. Many tRNA molecules have unusual nitrogenous bases, such as inosine, methylinosine, and dimethylguanine. These unconventional bases are created from the four conventional RNA nucleotides (A, G, U, and C) after transcription of the tRNA molecule has occurred.
- Where is the anticodon in a tRNA molecule?
- Where is the amino acid attached to a tRNA molecule?
The process of attaching an amino acid to a tRNA molecule is called charging. Amino acids are attached to the 3’ ends of tRNAs using enzymes called aminoacyl-tRNA synthetases. Specific features of aminoacyl-tRNA synthetases include the following:
- There are 20 different types of aminoacyl-tRNA synthetases per cell, one for each type of amino acid.
- Aminoacyl-tRNA synthetases are named for the amino acid that is attached to the tRNA. For example, alanyl-tRNA synthetase attaches the amino acid alanine to a tRNA molecule, while prolyl-tRNA synthetase attaches the amino acid proline to a tRNA molecule.
- Aminoacyl-tRNA synthetases do not attach the wrong amino acid to a tRNA very often. In fact, it is estimated that aminoacyl-tRNA synthetases only attach the wrong amino acid one time per 100,000 tRNAs charged.
The catalytic mechanism of an aminoacyl-tRNA synthetase is as follows (see figure 11.5):
- The aminoacyl-tRNA synthetase binds to its preferred amino acid and ATP.
- ATP is cleaved, pyrophosphate is released, and AMP is covalently linked to the amino acid forming an aminoacyladenylic acid When the aminoacyladenylic acid is produced, the aminoacyl-tRNA synthetase changes its conformation to produce the active form of the enzyme.
- The aminoacyl-tRNA synthetase binds to a tRNA molecule that contains an appropriate anticodon sequence. If the anticodon on the incoming tRNA is incorrect, the tRNA is released from the aminoacyl-tRNA synthetase and a new tRNA binds to the enzyme.
- The aminoacyl-tRNA synthetase attaches the amino acid to the 3’ end of the tRNA.
- AMP is released.
- The charged tRNA is released from the aminoacyl-tRNA synthetase.
- Which molecule provides the energy for the tRNA charging reaction?
- Explain the major events that occur during the tRNA charging reaction.
The Wobble Rules
Most synonymous codons (codons that encode the same amino acid) have identical bases at the first two nucleotide positions (the 5’-most base and the middle base). These first two bases obey the AU/GC base pairing rules in terms of codon-anticodon hydrogen bonding. Degeneracy in the genetic code typically occurs at the third base within the codon. The 3’-most nucleotide in the codon does not have to form conventional base pairing interactions with the 5’-most nucleotide of the anticodon, leading to “wobble.” The rules that govern codon-anticodon interactions are called the wobble rules (see Table 11.1).
A group of tRNA molecules, with slightly different anticodon sequences, which can recognize the same codon are called isoacceptor tRNAs. For example, suppose we have the codon 5’-UUU-3’. A tRNA containing either 3’-AAA-5’, 3’-AAG-5’, or 3’-AAI-5’ anticodons can recognize this codon, according to the wobble rules. Note that these isoacceptor tRNAs are charged by the same aminoacyl-tRNA synthetase and bear the same amino acid. Alternatively, a tRNA with the 3’-GGU-5’ anticodon can recognize two codons: 5’-CCA-3’and 5’-CCG-3’. Both codons encode proline.
Why would a cell use this wobble phenomenon? Wobble gives the cell tremendous flexibility in codon:anticodon interactions. A single codon can be recognized by more than one tRNA anticodon, and a single tRNA can bind to more than one codon. This flexibility allows a cell to save energy in the synthesis of tRNA molecules. Instead of assembling 61 different types of tRNA molecules (one type for each possible sense codon), a cell can get away with synthesizing only 30-40 types of tRNA molecules, with several tRNA types recognizing more than one codon.
- What is meant by wobble?
- What are isoacceptor tRNAs?
- What is the advantage of wobble?
A macromolecular machine called the ribosome is a central figure in the translation process. The ribosome binds to the mRNA, serves as a site for codon-anticodon recognition, breaks the covalent bond between the 3’ end of the tRNA and the amino acid, and synthesizes peptide bonds between amino acids.
A single prokaryotic cell is thought to contain 10,000 bacterial ribosomes (70S ribosomes). Note that the “S” designation of ribosomes is named after the scientist Theodor Svedberg and describes the sedimentation behavior of the ribosome during ultracentrifugation. The “S” designation correlates roughly with the overall three-dimensional shape of the ribosome component; the larger the molecule, the larger the “S” value.
The 70S ribosome consists of the following (see figure 11.6):
- Small subunit (30S subunit). The small subunit of the ribosome is composed of several different ribosomal proteins and a 16S ribosomal RNA (rRNA) The 16S rRNA molecule helps the ribosome bind to the bacterial mRNA molecule (see below).
- Large subunit (50S subunit). The large subunit of the ribosome is composed of many ribosomal proteins, a 5S rRNA, and a 23S rRNA The 23S rRNA molecule is the catalytic core of the ribosome; 23S rRNA is the ribozyme that forms peptide bonds between amino acids during translation (see below).
Eukaryotic cells have cytosolic ribosomes called 80S ribosomes. Mitochondrial and chloroplast ribosomes resemble prokaryotic ribosomes (70S).
The 80S ribosome consists of the following (see figure 11.7):
- Small subunit (40S). The 40S subunit is composed of many proteins and an 18S rRNA molecule.
- Large subunit (60S). The 60S subunit is composed of a collection of proteins, 5S, 5.8S, and 28S rRNA The 28S rRNA molecule is the ribozyme that forms peptide bonds between amino acids during translation (see below).
- Compare and contrast the structural features of prokaryotic vs. eukaryotic ribosomes.
- What is the name of the ribozyme within the 70S ribosome?
- What is the name of the ribozyme within the 80S ribosome?
Overview of Translation
Translation in prokaryotes and eukaryotes occurs in three stages (see figure 11.8):
- During initiation, the large and small ribosomal subunits assemble with the mRNA that is to be translated and with an initiator tRNA that has an anticodon specific for the start codon. Once all translation components have assembled correctly, the initiator tRNA is located at the P site (see below) within the ribosome.
- The ribosome moves or translocates along the mRNA in the 5’ to 3’ direction, reading the nucleic acid sequence of the mRNA as triplet codons, converting the nucleic acid sequence into a polypeptide chain. During elongation, three tRNA binding sites within the assembled ribosome are important for protein synthesis. These tRNA binding sites are the:
- Aminoacyl site (A site). The A site is the binding site for charged tRNAs that enter the ribosome.
- Peptidyl site (P site). The polypeptide chain attached to the tRNA molecule is found in the P site of the ribosome. The polypeptide is transferred from the P site to the A site when the peptide bond forms (see below).
- Exit site (E site). The E site is the exit site for uncharged tRNA molecules.
- When the ribosome reaches a stop codon, translation ends. The mRNA and the polypeptide chain are released from the ribosome, and the large and small subunits of the ribosome dissociate from each other.
- What major events are occurring during the initiation, elongation, and termination stages of translation?
- What are the functions of the A site, the P site, and the E site?
The 5’-untranslated region (5’-UTR) of prokaryotic mRNAs plays a critical role in translation; the 5’-UTR contains the ribosome-binding site. Specifically, the 5’-UTR contains a recognition sequence (5’-AGGAGGU-3’) called the Shine-Dalgarno sequence that forms hydrogen bonds with a complementary sequence within the 16S rRNA of the small ribosomal subunit (30S) (see figure 11.9). Hydrogen bond formation between the Shine-Dalgarno sequence and the 16S rRNA promotes ribosome assembly. Further, the Shine-Dalgarno sequence positions the P site of the ribosome properly at the start codon to initiate polypeptide synthesis.
- What is the function of the Shine-Dalgarno sequence?
Translation Initiation in Bacteria
Translation initiation in the bacterium E. coli includes the following (see figure 11.10):
- The mRNA binds to the small ribosomal subunit (30S) via the Shine-Dalgarno sequence. An initiation factor protein called IF3 promotes the formation of hydrogen bonds between the ribosome and the mRNA.
- An initiator tRNA recognizes the start codon within the P site of the ribosome. The initiator tRNA anticodon forms hydrogen bonds with the start codon in the P site. In bacteria, this initiator tRNA is covalently attached to a modified form of methionine called N-formylmethionine (fmet). Thus, the N-terminal amino acid in all newly synthesized prokaryotic proteins is fmet. Binding of the tRNAfmet to the start codon requires another initiation factor protein called IF2 that cleaves GTP. The complex of the 30S subunit, the mRNA, IF3, and IF2 is called the initiation complex.
- The 50S subunit of the ribosome is added to the initiation complex. The addition of 50S requires the release of IF2 and IF3.
- How do IF2 and IF3 function in translation initiation in bacteria?
- Which nucleotide provides the energy for translation initiation in bacteria?
Translation Initiation in Eukaryotes
Translation initiation is more complex in eukaryotes than in prokaryotes. Key differences between eukaryotic and prokaryotic translation initiation include (see figure 11.11):
- More initiation factor proteins are involved. These proteins are called eukaryotic initiation factors (eIFs).
- The initiator tRNA is tRNAmet. This initiator tRNA is charged with the amino acid methionine.
- There is no Shine-Dalgarno sequence in a eukaryotic mature mRNA. Instead, eukaryotic mature mRNAs contain a 7-methylguanosine (7-mG) cap and a Kozak sequence within the 5’-UTR.
The process of translation initiation in eukaryotes is as follows:
- eIF2 binds to tRNAmet and guides it to the 40S ribosomal subunit. eIF2 cleaves GTP to accomplish this function.
- Assembly of a multiprotein initiation complex on the 7-methylguanosine (7-mG) cap at the 5’ end of the mRNA. This initiation complex includes the following:
- 40S ribosome subunit
- A complex composed of eIF2 and tRNAmet
- An eIF4 protein complex.
- Cap-binding protein 1 (CBP1). CBP1 is the protein responsible for recognizing the 7-mG structure on eukaryotic mature mRNAs.
- Identification of the start codon. The 40S ribosomal subunit and associated factors start at the 7-mG cap and move 5’ to 3’ along the mRNA looking for a start codon. This scanning process requires the consumption of ATP. Not all possible start codons (5’-AUG-3’) are chosen to initiate translation. The 5’-AUG-3’ that is chosen is within the consensus sequence 5’- GCC (A or G)CCAUGG – 3’. This consensus sequence is called the Kozak sequence. When the tRNAmet interacts with the start codon, tRNAmet is in the P site of the ribosome.
- Addition of the 60S ribosomal subunit to form the active 80S ribosome.
Figure 9.11 - Translation initiation in eukaryotes
- How do eIF2 and CBP1 function in translation initiation in eukaryotes?
- What is the function of the Kozak sequence?
- Which two nucleotides provide the energy for translation initiation in eukaryotes?
Translation elongation in bacteria involves a series of cycles, one cycle per sense codon in the mRNA. At the beginning of each cycle, a growing polypeptide chain attached to a tRNA molecule is located within the P site of the ribosome (see figure 11.12). The A and E sites of the ribosome are empty. Each elongation cycle then proceeds as follows:
- A charged tRNA binds to the A site. A prokaryotic elongation factor protein called EF-Tu is responsible for loading a charged tRNA into the A site of the ribosome. This loading of a charged tRNA into the A site requires the cleavage of GTP.
- The codon and anticodon hydrogen bonds are checked. Of course, the anticodon within the newly delivered tRNA must be complementary to the mRNA codon according to the wobble rules. An incorrect tRNA bound to the A site is recognized by the 16S rRNA. When an incorrect anticodon is encountered, polypeptide synthesis is halted until the mismatched tRNA is released. This editing function of the 16S rRNA allows for high fidelity in protein synthesis; it is estimated that only one mistake is made per 10,000 incorporated amino acids, or about one time in every 20 polypeptides synthesized by the ribosome.
- The formation of a peptide bond. The polypeptide chain bound to the tRNA in the P site is transferred to the tRNA in the A site via the formation of a new peptide bond. This enzymatic reaction is catalyzed by the 23S rRNA component of the 50S subunit. As a result, the 23S rRNA is often called peptidyl transferase. The 23S rRNA is an example of a ribozyme (catalytic RNA molecule).
- Ribosome translocation. The ribosome moves (translocates) in the 5’ to 3’ direction one codon along the mRNA. The tRNA (with the attached polypeptide chain) that was in the A site moves to the P site. The uncharged tRNA that was in the P site moves to the E site and exits the ribosome. A bacterial elongation factor protein called EF-G cleaves GTP to translocate the ribosome to the next sense codon in the mRNA.
- Steps 1–4 are repeated for each sense codon in the mRNA. The rate of protein synthesis is 15–18 amino acids per second in prokaryotes.
Translation elongation works similarly in both prokaryotes and eukaryotes. Below are some differences to keep in mind for eukaryotic translation:
- The 28S rRNA of the 60S ribosomal subunit functions as the peptidyl transferase (synthesizes the peptide bonds between amino acids during translation). The 28S rRNA is another example of a ribozyme.
- Eukaryotes have eukaryotic elongation factors (eEFs) that function very similarly to EF-Tu and EF-G from bacteria. Eukaryotic eEF1 functions similarly to EF-Tu in bacteria, delivering charged tRNA molecules to the A site of the ribosome. Eukaryotic eEF2 functions similarly to EF-G in bacteria, moving the ribosome one codon towards the 3’ end of the mRNA after the peptidyl transferase reaction has taken place. Both eEF1 and eEF2 cleave GTP molecules.
- Describe the events involved in one translation elongation cycle.
- Explain the steps where GTP is used as an energy source.
- What is the eukaryotic equivalent of 23S rRNA, EF-Tu, and EF-G?
Translation termination in bacteria occurs as follows (see figure 11.13):
- As the ribosome approaches the 3’ end of the mRNA, a stop codon (5’-UGA-3’, 5’-UAG-3’, or 5’-UAA-3’) becomes located in the A site of the ribosome.
- The stop codon is recognized by a type of translation factor protein called a release factor. A release factor protein mimics the three-dimensional structures of tRNA molecules. In bacteria, the release factor 1 (RF1) protein recognizes the stop codons 5’-UAA-3’ and 5’-UAG-3’. The release factor 2 (RF2) protein recognizes 5’-UAA-3’ and 5’-UGA-3’.
- The release factor protein cleaves the covalent bond between the tRNA and the polypeptide chain in the ribosome P site. The tRNA and the polypeptide chain are released from the ribosome. This step consumes GTP.
- The ribosomal subunits, mRNA, and release factor protein dissociate, terminating translation.
Translation termination works essentially the same way in eukaryotes. In terms of the termination step in eukaryotes, a single release factor protein called eukaryotic release factor (eRF) recognizes all three possible stop codons. eRF consumes GTP to terminate translation.
- What is the difference between RF1 and RF2?
- What nucleotide provides the energy to terminate translation?
- Discuss what happens to each of the individual molecules after translation is terminated. Are some of them recycled? If so, which ones?
Polysomes and Coupled Transcription and Translation
In the electron microscope, many ribosomes (polyribosomes or polysomes) can be observed, attached to a single mRNA transcript (see figure 11.14). Polysomes are visualized attached to both prokaryotic mRNAs and eukaryotic mature mRNA transcripts.
Since prokaryotic cells lack nuclei, transcription and translation in bacteria are coupled, meaning that translation can begin before transcription is completed. In eukaryotes, transcription and translation are uncoupled. Transcription in eukaryotes occurs within the nucleus; translation occurs later, in the cytoplasm.
- Why is it advantageous to have multiple ribosomes working simultaneously?
- Why is prokaryotic transcription and translation coupled?
- Why is eukaryotic transcription and translation uncoupled?
Fill in the blanks:
- The ______________ portion of a eukaryotic mature mRNA contains the polyadenylation signal sequences.
- The genetic code is said to be __________________, which means that each codon only specifies one type of amino acid. (For example, 5’-UUU-3’ always encodes phenylalanine)
- The cloverleaf structure of the tRNA is such that the ____________________________ is a single-stranded loop that forms hydrogen bonds with the mRNA codon, and the ___________________________ is the single-stranded region where an amino acid is attached via a covalent linkage.
- ____________________________ is an enzyme that charges the amino acid to a tRNA.
- The 30S and 50S ribosomal subunits in a prokaryotic cell combine to form a final size of _________ while in eukaryotes the _________ and __________ ribosomal subunits combine to form a final size of ______________.
- The assembled ribosome contains three tRNA binding sites. The site in which a tRNA carrying a single amino acid resides is called the _____ site.
- The initiator tRNA binds to the __________ site of the ribosome, whereas the remaining charged tRNAs bind to the ___________ site of the ribosome.
- An E. coli protein called _______________ delivers charged tRNAs to the ribosome during translation elongation.
- The ribosome moves along the mRNA in the _______to _______ direction.
- _______________ is a eukaryotic translation factor protein that delivers charged tRNAs to the ribosome during elongation.
- ___________ is a eukaryotic translation factor that helps the ribosome move one codon in the 3’ direction along the mRNA.
- The _________________ protein in bacteria mimics the structure of a tRNA and recognizes 5’-UGA-3’ in the A site of the ribosome.
- An E. coli protein called __________ delivers tRNAfmet to the ribosome during translation.
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