Bacterial Translation Help
No membrane separates the DNA from the ribosomes in a bacterial cell. Hence, as soon as the 5' end of an RNA is transcribed, ribosomes can begin translation. In other words, transcription and translation are coupled processes in bacteria.
Translation occurs in three major steps: (1) initiation, (2) elongation, and (3) termination.A nucleotide sequence called the Shine-Dalgarno sequence (AGGAGG) in the leader of an mRNA molecule is complementary to a sequence at the 3' end of 16S rRNA and serves as a binding site for ribosomes.The initiation codon near the 5' end of an mRNA molecule is 5'-AUG, coding for the amino acid methionine. In bacteria, a formyl group (CHO) becomes attached to the amino group of the methionine after it has become attached to its tRNA molecule. A deformylase enzyme removes the formyl groups from some polypeptide chains soon after their synthesis commences. In other cases, anaminopeptidase enzyme removes the terminal methionine (or part of the amino terminal end), so that not all functional bacterial proteins have formyl-methionione or methionine at their N termini.
Initiation of protein synthesis begins with the formation of a complex involving the 30S ribosomal subunit, guanosine triphosphate (GTP), and three protein initiation factors (IF1, IF2, IF3). In the next step, formylated methionyl-tRNA and them RNA attach to the IF-30S-GTPcomplex, forming a 30S initiation complex. Then IF3 is released, the 50S subunit is added, GTP is hydrolyzed, and IF1 and IF2 are released. The final complex is called a 70S initiation complex. Only the initiator region of the mRNA can simultaneously form two sets of base-pairing interactions (16S rRNA-mRNA and mRNA-fMet tRNA). In this way, the AUG start codon is distinguished from other AUG codons downstream in the mRNA. Base pairing between the leader of the mRNA and the 16S RNA dissociates after formation of the 70S initiation complex, so that elongation can begin.
1 S = Svedberg unit; a sedimentation coefficient for molecules in an ultracentrifuge. The S value tends to increase with the molecular weight of the molecule, but the geometry of the molecule also may be influential. Note that S units are not additive; i.e.; 50S subunitþ30S subunit¼ 70S for the complete bacterial ribosome, not 80S.
The elongation phase requires GTP, three protein elongation factors (EF-Tu, EF-Ts, and EF-G), and a peptidyl transferase activity (carried out by catalyticRNAmolecules—ribozymes—within the ribosome). Two molecules of GTP are hydrolyzed for each amino acid added to the growing polypeptide chain. EF-Tu and EF-Ts cyclically interact to align each amino-acyl-tRNA complex (AA-tRNA) for effective codon-anticodon base pairing. EF-G (also called translocase) and GTP form a complex that mediates the movement of peptidyl-tRNA fromthe A site to the P site. Hydrolysis of GTP to guanosine diphosphate (GDP) is required for entry of the ribosome into the next elongation cycle. Several ribosomes maysimultaneously translate the samemRNA, resulting in a structure called a polyribosome.
Termination of translation in E. coli requires at least two protein release factors. RF1 recognizes the mRNA stop codonsUAGandUAA; RF2 recognizesUGAandUAA. These RFs cause peptidyl transferase to transfer the completed polypeptide chain to water instead of to an AA-tRNA. After chain termination, the 30S and 50S ribosomal subunits dissociate from the mRNA and are then free to recycle into new initiation complexes on the same or different mRNA templates.
If a translational product is to be transported across the plasma membrane to the cell's exterior, the protein usually contains 15–30 additional amino acids (called a signal sequence) at its N terminus. The signal sequence is rich in uncharged, usually hydrophobic, amino acids that become anchored in the membrane, whereas the remainder of the polypeptide chain is extruded through the membrane as it is synthesized. A signal peptidase then cleaves the signal sequence to release the protein from the cell.
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