9. As the ribosome moves by two codons, next round of protein synthesis is initiated by the attachment of a new ribosome. Thus, at a time, a single mRNA is found to be attached with many ribosomes with their polypeptides of different length, (shortest polypeptide at the 5′ end of the mRNA and longest at the 3′ end), called polysomes.
Fig. 8.15 Peptide bond formation in growing polypeptide.
10. Ultimately, the A-site of ribosome is occupied by the termination codon (UAA,UAG or UGA) at the 3′ end of mRNA, which is not recognized by any tRNA. Thus, the termination of the protein synthesis is helped by the release factors RFl, RF2 and RF3 (in eukaryotes eRF1), which release the newly synthesized polypeptide chain from the P-site (Fig. 8.16).
The tRNA that recognizes the AUG start codon and the amino acid attached to it are also distinct from the Met-tRNAMet base-paired to internal AUG codons. This initiator tRNA contains the CAU anticodon necessary for recognition of the start codon, but several sequence and structural differences distinguish it from the elongator tRNA that inserts methionine into internal positions of the polypeptide. In bacteria, both elongator and initiator tRNAs are aminoacylatedby methionyl-tRNA synthetase, but the methionylated initiator tRNA undergoes further processing prior to its transport to the ribosome. The enzyme fMet-tRNA transformylase modifies the amino group of the tRNA-attached methionine residue, using N10-formyltetrahydrofolate as a formyl donor. One feature that the transformylase enzyme recognizes is a mismatched base pair at the first position of the initiator tRNA (tRNAfMet) acceptor stem. The elongator tRNA (tRNAMet) molecule has a canonical G:C base pair at the first position of the acceptor stem, while tRNAfMet contains a C:A pair. Base substitutions that produce a strong base pair at this position in tRNAfMet significantly decrease formylation of Met-tRNAfMet.
Fig. 8.16 Translation process of protein synthesis in prokaryotes.
A combination of nucleotide signals identifies the beginning of an mRNA sequence to be translated into its protein product. The nucleotide triple AUG is the start codon that directs the ribosome to begin reading an mRNA and orients the message in the right frame (for example,… CUA GUG CAC C… rather than … C UAG UGC ACC…, which would be a different protein). However, AUG is also the codon for insertion of the amino acid methionine into the body of the polypeptide chain. What distinguishes the start AUG from other identical codons elsewhere in the message? A stretch of 3-10 nucleotides located about 10 nucleotides upstream (in the 5′-direction) from the start codon is called the Shine-Dalgarno sequence, after the researchers who identified it. This sequence is rich in A and G nucleotides, and is partially complementary to a short region of U and C nucleotides near the 3′-end of an RNA molecule embedded within the ribosomal small sub-unit. Such complementarity positions the incoming message properly on the ribosome, so that the start codon is in the ribosomal decoding site for initiation of protein synthesis. Once translation begins, the rest of the codons in the message need to be read, so the base-pairing interaction between mRNA and rRNA at the Shine-Dalgarno sequence must be transient.
Protein Synthesis - Protein Synthesis
N2 - The mechanisms of action of cycloheximide and emetine in exponentially growing Chinese hamster ovary cells have been studied (a) by measuring the incorporation of [35S]methionine into N-terminal and internal positions of nascent peptide chains by an Edman degradation modified for handling a large number of samples and (b) by determining the size distribution of polysomes in the presence of the drugs. These combined techniques have confirmed that all three phases of protein synthesis (initiation, elongation, and termination) are susceptible to inhibition by cycloheximide and have shown that the primary sensitive step varies with the concentration of drug between 10-9 and 10-3 m. At the lowest doses, initiation appears, by these criteria, to be the step most sensitive to inhibition by cycloheximide, while emetine seems to act primarily on elongation.
Protein Synthesis and Degradation | Basicmedical Key
Decoding of the protein message occurs at the ribo-some, after prior attachment of amino acids to the tRNA molecules. Aminoacyl-tRNA synthetases (AARSs) are the family of enzymes responsible for covalent attachment of each amino acid to its correct, or cognate, tRNA molecule. This first step in protein synthesis is responsible for establishing the rules of […]
Chapter 13 Protein Synthesis and Degradation Tracy G
In eukaryotes, the termination sites in DNA are present far away from the corresponding actual 3′ end of mRNA, thus, to produce HnRNA (heterogeneous nuclear RNA). The 3′ end of the mRNA is generated after the processing of HnRNA by snurp (small nuclear RNA-protein complex). In addition to these extra nucleotides at 3′ end, the HnRNA may also contain extra nucleotide sequences at the 5′ end and at the internal positions. These extra nucleotide sequences at internal positions are called introns, whereas, the nucleotide sequences in between the introns