Protein Synthesis: Translation of the Genetic Message

Chapter Twelve

Translating the Genetic Message

• Protein biosynthesis is a complex process requiring ribosomes, mRNA, tRNA, and protein factors

• Several steps are involved

• Before being incorporated into growing protein chain, a.a. must be activated by tRNA and aminoacyl-tRNA synthetases

Salient features of the genetic code

– Triplet: a sequence of three bases (a codon) is needed to specify one amino acid

– Nonoverlapping: no bases are shared between consecutive codons

– Commaless: no intervening bases between codons– Degenerate: more than one triplet can code for

the same amino acid; Leu, Ser, and Arg, for example, are each coded for by six triplets

– Universal: the same in viruses, prokaryotes, and eukaryotes

The Genetic Code

• The ribosome moves along the mRNA three bases at a time rather than one or two at a time

The Genetic Code

• All 64 codons have assigned meanings

– 61 code for amino acids

– 3 (UAA, UAG, and UGA) serve as termination signals

– only Trp and Met have one codon each

The Genetic Code

• The third base is irrelevant for Leu, Val, Ser, Pro, Thr, Ala, Gly, and Arg

• Amino acids coded by 2, 3, or 4 triplets - the third letter of the codon - Gly, for example, is coded by GGA, GGG, GGC, and GGU

Wobble Base Pairing

• Some tRNAs bond to one codon exclusively, but many tRNAs can recognize more than one codon because of variations in allowed patterns of hydrogen bonding– the variation is called

“wobble”– wobble is in the first

base of the anticodon

Base Pairing Combination in the Wobble Scheme

If there are 64 codons, how can there be less than 64 tRNA molecules?

• The wobble hypothesis provides insight– in many cases, the degenerate codons for a given

amino acid differ only in the third base; therefore fewer different tRNAs are needed because a given tRNA can base-pair with several codons

– the existence of wobble minimizes the damage that can be caused by a misreading of the code; for example, if the Leu codon CUU were misread as CUC or CUA or CUG during transcription of mRNA, the codon would still be translated as Leu during protein synthesis

Amino Acid Activation• Amino acid activation

and formation of the aminoacyl-tRNA take place in two separate steps

• Both catalyzed by aminoacyl-tRNA synthetase

• Free energy of hydrolysis of ATP provides energy for bond formation

tRNA Tertiary Structure

• There are several recognition sites for various amino acids on the tRNA

Chain Initiation

• In all organisms, synthesis of polypeptide chain starts at the N-terminal end, and grows from N-terminus to C-terminus

• Initiation requires:– tRNAfmet

– initiation codon (AUG) of mRNA– 30S ribosomal subunit– 50S ribosomal subunit– initiation factors IF-1, IF-2, and IF-3– GTP, Mg2+

• Forms the initiation complex

The Initiation Complex

Chain Initiation

– tRNAmet and tRNAfmet contain the triplet 3’-UAC-5’– Triplet base pairs with 5’-AUG-3’ in mRNA

– 3’-UAC-5’ triplet on tRNAfmet recognizes the AUG triplet (the start signal) when it occurs at the beginning of the mRNA sequence that directs polypeptide synthesis

– 3’-UAC-5’ triplet on tRNAmet recognizes the AUG triplet when it is found in an internal position in the mRNA sequence

How does the ribosome know where to start translating

– Start signal is preceded by a Shine-Dalgarno purine-rich leader segment, 5’-GGAGGU-3’

– Lies about 10 nucleotides upstream of the AUG start signal and acts as a ribosomal binding site

Chain Elongation

• Uses three binding sites for tRNA present on the 50S subunit of the 70S ribosome: P (peptidyl) site, A (aminoacyl) site, E (exit) site.

• Requires– 70S ribosome– codons of mRNA– aminoacyl-tRNAs– elongation factors EF-Tu (Elongation factor temperature-

unstable), EF-Ts (Elongation factor temperature-stable), and EF-G (Elongation factor-GTP)

– GTP, and Mg2+

Chain Elongation

Why is EF-Tu so important in E.coli?

• Involved in translation fidelity• tRNA and aminoacid are mismatched then EF-

Tu will not bind to t-RNA and will not deliver it to ribosome

• Binds activated tRNA too well and does not release it from ribosome

Elongation Steps• Step 1

– an aminoacyl-tRNA is bound to the A site– the P site is already occupied– 2nd amino acid bound to 70S initiation complex. Defined by the mRNA

• Step 2– EF-Tu is released in a reaction requiring EF-Ts

• Step 3– the peptide bond is formed, the P site is uncharged

• Step 4– the uncharged tRNA is released– the peptidyl-tRNA is translocated to the P site– EF-G and GTP are required– the next aminoacyl-tRNA occupies the empty A site

Chain Termination

• Chain termination requires– stop codons (UAA, UAG, or UGA) of mRNA– RF-1 (Release factor-1) which binds to UAA and UAG or

RF-2 (Release factor-2) which binds to UAA and UGA– RF-3 which does not bind to any termination codon, but

facilitates the binding of RF-1 and RF-2– GTP which is bound to RF-3

• The entire complex dissociates setting free the completed polypeptide, the release factors, tRNA, mRNA, and the 30S and 50S ribosomal subunits

Chain Termination

Components of Protein Synthesis

Protein Synthesis

– In prokaryotes, translation begins very soon after mRNA transcription

– It is possible to have several molecules of RNA polymerase bound to a single DNA gene, each in a different stage of transcription

– It is also possible to have several ribosomes bound to a single mRNA, each in a different stage of translation

– Polysome: mRNA bound to several ribosomes– Coupled translation: the process in which a prokaryotic

gene is being simultaneously transcribed and translated

Simultaneous Protein Synthesis on Polysomes

• A single mRNA molecule is translated by several ribosomes simultaneously

• Each ribosome produces a copy of the polypeptide chain specified by the mRNA

• When protein has been completed, the ribosome dissociates into subunits that are used again in protein synthesis

Simultaneous Protein Synthesis on Polysomes

Eukaryotic Translation• Chain Initiation:

• the most different from process in prokaryotes• 13 more initiation factors are given the designation eIF (eukaryotic initiation factor) (Table 12.4)

Eukaryotic Translation

• Chain elongation– uses the same mechanism of peptidyl transferase and

ribosome translocation as prokaryotes– there is no E site on eukaryotic ribosomes, only A and P sites– there are two elongation factors, eEF-1 and eEF-2– eEF2 is the counterpart to EF-G, which causes translocation

• Chain termination– stop codons are the same: UAG, UAA, and UGA– only one release factor that binds to all three stop codons

Posttranslational Modification

• Newly synthesized polypeptides are frequently modified before they reach their final form where they exhibit biological activity– N-formylmethionine in prokaryotes is cleaved– leader sequences are removed by specific proteases of the

endoplasmic reticulum; the Golgi apparatus then directs the finished protein to its final destination

– factors such as heme groups may be attached– disulfide bonds may be formed– amino acids may be modified, as for example, conversion of

proline to hydroxyproline– other covalent modifications; e.g., addition of carbohydrates

Examples of Posttranslational Modification

Protein Degradation

• Degradative pathways are restricted to – subcellular organelles such as lysosomes– macromolecular structures called proteosomes

Protein Degradation

• In eukaryotes, ubiquitinylation (becoming bonded to ubiquitin) targets a protein for destruction– those with an N-terminus

of Met, Ser, Ala, Thr, Val, Gly, and Cys are resistant

– those with an N-terminus of Arg, Lys, His, Phe, Tyr, Trp, Leu, Asn, Gln, Asp, Glu have short half-lives

Acidic N-termini Induced Protein Degradation

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